The teacher is doubtful about having parents working as classroom assistants.

doubtful 是形容词，由名词 doubt（怀疑）加上后缀 -ful（充满...的）构成，意思是"充满怀疑的、不确定的"。 在句中表示老师对家长当助教这件事持怀疑态度，内心不确定这种做法是否合适。