public class Course
{
    public Course(string name, Teacher teacher)
    {
        Name = name;
        CourseTeacher = teacher;
    }

    public string Name { get; private set; }
    public Teacher CourseTeacher { get; private set; }
}
public class Teacher
{
    public Teacher(string name)
    {
        Name = name;
    }

     public string Name { get; private set; }
}
public class SubstituteTeacher : Teacher
{
    public SubstituteTeacher(string name) : base(name)
    {
    }
}

var course = new Course("Math", new SubstituteTeacher("Larry Smith"));
await collection.InsertOneAsync(course);

System.ArgumentOutOfRangeException
 HResult=0x80131502
 Message=The memberInfo argument must be for class SubstituteTeacher, but was for class Teacher.
Parameter name: memberInfo
 Source=MongoDB.Bson
 StackTrace:
 at MongoDB.Bson.Serialization.BsonClassMap.EnsureMemberInfoIsForThisClass(MemberInfo memberInfo)
 at MongoDB.Bson.Serialization.BsonClassMap.MapMember(MemberInfo memberInfo)
 at MongoDB.Bson.Serialization.Conventions.ImmutableTypeClassMapConvention.Apply(BsonClassMap classMap)
 at MongoDB.Bson.Serialization.Conventions.ConventionRunner.Apply(BsonClassMap classMap)
 at MongoDB.Bson.Serialization.BsonClassMap.AutoMapClass()
 at MongoDB.Bson.Serialization.BsonClassMap.LookupClassMap(Type classType)
 at MongoDB.Bson.Serialization.BsonClassMapSerializationProvider.GetSerializer(Type type, IBsonSerializerRegistry serializerRegistry)
 at MongoDB.Bson.Serialization.BsonSerializerRegistry.CreateSerializer(Type type)
 at System.Collections.Concurrent.ConcurrentDictionary`2.GetOrAdd(TKey key, Func`2 valueFactory)
 at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.Serialize(BsonSerializationContext context, BsonSerializationArgs args, TClass value)
 at MongoDB.Bson.Serialization.Serializers.SerializerBase`1.MongoDB.Bson.Serialization.IBsonSerializer.Serialize(BsonSerializationContext context, BsonSerializationArgs args, Object value)
 at MongoDB.Bson.Serialization.IBsonSerializerExtensions.Serialize(IBsonSerializer serializer, BsonSerializationContext context, Object value)
 at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.SerializeClass(BsonSerializationContext context, BsonSerializationArgs args, TClass document)
 at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.Serialize(BsonSerializationContext context, BsonSerializationArgs args, TClass value)
 at MongoDB.Bson.Serialization.Serializers.SerializerBase`1.MongoDB.Bson.Serialization.IBsonSerializer.Serialize(BsonSerializationContext context, BsonSerializationArgs args, Object value)
 at MongoDB.Bson.Serialization.IBsonSerializerExtensions.Serialize(IBsonSerializer serializer, BsonSerializationContext context, Object value)
 at MongoDB.Bson.Serialization.Serializers.BsonValueSerializerBase`1.Serialize(BsonSerializationContext context, BsonSerializationArgs args, TBsonValue value)
 at MongoDB.Bson.Serialization.Serializers.SerializerBase`1.MongoDB.Bson.Serialization.IBsonSerializer.Serialize(BsonSerializationContext context, BsonSerializationArgs args, Object value)
 at MongoDB.Bson.Serialization.IBsonSerializerExtensions.Serialize(IBsonSerializer serializer, BsonSerializationContext context, Object value)
 at MongoDB.Bson.Serialization.Serializers.BsonValueSerializerBase`1.Serialize(BsonSerializationContext context, BsonSerializationArgs args, TBsonValue value)
 at MongoDB.Driver.Core.Operations.BulkInsertOperation.InsertRequestSerializer.SerializeValue(BsonSerializationContext context, BsonSerializationArgs args, InsertRequest value)
 at MongoDB.Bson.Serialization.Serializers.SealedClassSerializerBase`1.Serialize(BsonSerializationContext context, BsonSerializationArgs args, TValue value)
 at MongoDB.Bson.Serialization.Serializers.SerializerBase`1.MongoDB.Bson.Serialization.IBsonSerializer.Serialize(BsonSerializationContext context, BsonSerializationArgs args, Object value)
 at MongoDB.Driver.Core.WireProtocol.Messages.Encoders.BinaryEncoders.CommandMessageBinaryEncoder.WriteType1Section(BsonBinaryWriter writer, Type1CommandMessageSection section, Int64 messageStartPosition)
 at MongoDB.Driver.Core.WireProtocol.Messages.Encoders.BinaryEncoders.CommandMessageBinaryEncoder.WriteSections(BsonBinaryWriter writer, IEnumerable`1 sections, Int64 messageStartPosition)
 at MongoDB.Driver.Core.WireProtocol.Messages.Encoders.BinaryEncoders.CommandMessageBinaryEncoder.WriteMessage(CommandMessage message)
 at MongoDB.Driver.Core.Connections.BinaryConnection.SendMessagesHelper.EncodeMessages(CancellationToken cancellationToken)
 at MongoDB.Driver.Core.Connections.BinaryConnection.<SendMessagesAsync>d__58.MoveNext()
 at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
 at MongoDB.Driver.Core.WireProtocol.CommandUsingCommandMessageWireProtocol`1.<ExecuteAsync>d__13.MoveNext()
 at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
 at MongoDB.Driver.Core.Servers.Server.ServerChannel.<ExecuteProtocolAsync>d__29`1.MoveNext()
 at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
 at MongoDB.Driver.Core.Operations.RetryableWriteOperationExecutor.<ExecuteAsync>d__3`1.MoveNext()
 at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
 at MongoDB.Driver.Core.Operations.BulkUnmixedWriteOperationBase`1.<ExecuteBatchAsync>d__46.MoveNext()
 at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
 at MongoDB.Driver.Core.Operations.BulkUnmixedWriteOperationBase`1.<ExecuteBatchesAsync>d__48.MoveNext()
 at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
 at MongoDB.Driver.Core.Operations.BulkMixedWriteOperation.<ExecuteBatchAsync>d__51.MoveNext()
 at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
 at MongoDB.Driver.Core.Operations.BulkMixedWriteOperation.<ExecuteAsync>d__44.MoveNext()
 at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
 at MongoDB.Driver.OperationExecutor.<ExecuteWriteOperationAsync>d__5`1.MoveNext()
 at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
 at MongoDB.Driver.MongoCollectionImpl`1.<ExecuteWriteOperationAsync>d__95`1.MoveNext()
 at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
 at MongoDB.Driver.MongoCollectionImpl`1.<BulkWriteAsync>d__26.MoveNext()
 at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
 at MongoDB.Driver.MongoCollectionImpl`1.<UsingImplicitSessionAsync>d__101`1.MoveNext()
 at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
 at MongoDB.Driver.MongoCollectionBase`1.<InsertOneAsync>d__70.MoveNext()
 at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
 at JohnKnoop.MongoRepository.ManualTesting.Program.<Main>d__0.MoveNext() in C:\work\JohnKnoop.MongoRepository.ManualTesting\Program.cs:line 132

public class SubstituteTeacher : Teacher {
    public SubstituteTeacher(string name) : base(name) {
    }
    
    public string AdditionalProperty { get; private set; }
}