Is a teacher allowed to do this?

She isn't at liberty to take your phone even thou she did you could take it to your parents if not the principle to get your phone back you should not use your phone in class this is her time, not yours she is wrong for confiscating your phone.

I personally think no. If the events in the past were not bad, no, she cannot take your phone. and I think she invaded your personal space. You need(or want) to confront her and say (not in exact words, but something simular) "Mrs(name), you don't have permission to take my phone. You just asked to see one picture. Why did you scroll through all of my pictures when you only wanted to see one? That's invading my personal space. You DO NOT have permisson to do this. Even if you're a teacher, you can't. Some of the pictures were very personal to me too. It hurt my feelings."
If she is nice, she might ask you a few more questions and give it back.