You mandarin teacher just too ignorant to tell about different region Chinese language usage custom, and fail on learning the importance of presenting language in explicitly ways. Perhaps I will say , "for the sake of correcting to correct".
You can write it, but please don't use "我姨" in speaking, unless you are comfortable to the regions custom that use that(perhaps Beijing?).
Mandarin depends heavily in vocal rhythm changes, many verbs shared the same vocal rhythm. It is bad in vocal saying "我姨", because the vocal "姨" shared with different meaning vocals(like 已，咦， 宜, 疑,etc）
So Chinese already invented the sharp vocal "Ar" as people calling reference, thus when you speak "我阿姨”， all Chinese in different region will understand you. You will get a blank face when you say "我姨", even native mandarin speaker will have trouble fetch the meaning.
Now, back to "我親阿姨", it is quite redundant in Chinese speaking custom. Normally, just say/write "我阿姨" is enough. Chinese always treat 阿姨 as close relative.
When come to 阿姨, Chinese only explicitly define ladies who is NOT your relative, for example, the housekeeper 管家阿姨, the lady stay at the floor below you 楼下的阿姨。