As you noticed this doesn't work in 2.10 for some reason. I ran into the same with the avro source. Try changing to:
override def hdfsScheme = (new ParquetTupleScheme(fields)).asInstanceOf[Scheme[JobConf, RecordReader[_, _], OutputCollector[_, _], _, _]]
which is exactly what HadoopSchemeInstance does. No idea why this works, it would probably be good to know.