先生に日本語を教えて___のは失礼になることがあります。
Q. Which benefactive can sound condescending toward superiors?
Translation
Saying you will 'teach' your teacher Japanese with てあげる can sound rude.
Explanation
〜てあげる frames the action as a favor you bestow, which can sound patronizing toward superiors. With teachers or bosses, humble constructions (お教えする etc.) or plain forms are safer.