There was a student who was playing with his cell phone during the class without the teacher's seeing.
こんにちは！
ご質問ありがとうございます。
『授業中先生に隠れてケータイをいじっている人がいた』は、
いくつか言い方が考えられますので、ここでは一例を挙げておきますね！
There was a student who was playing with his cell phone during the class without the teacher's seeing. として、
『先生の目を盗んで授業中に携帯で遊んでいる生徒がいました。』と言えます。
メモ
play with ～で遊ぶ
during the class 授業中
without someone's seeing 人の目を盗んで
参考になれば幸いです。
One of the students was playing on his phone behind the teacher's back during the class.
ーOne of the students was playing on his phone behind the teacher's back during the class.
「授業中、先生に隠れて携帯をいじっていた生徒がいた」
to play on one's phone で「携帯をいじる」
behind the teacher's back は直訳すると「先生の背中の後ろで」となり「先生に隠れて」と訳せます。
during the class で「授業中に」
ご参考まで！
日本 
カナダ 
