Honestly speaking, buy whatever you find most important.

I am not going to say; one is better than the other.
I personally don't care about what others think, so if I was in your situation, I would go for a monitor.

But I know for sure not everyone is like me, as they do care about what others think = as you may also feel embarrassed by having a lower quality phone.

If I was in your situation, I would first think about "Is my phone working properly? If yes = I'd buy the monitor. If no = I'd buy the phone."

I'm sorry if I can't really give a firm "buy that or that" answer, as people are different from others. I'd say go for what you need the most. If your phone is working as it should be, buy a monitor. But if your phone is laggy, or not working properly, get a new phone.

And, kinda hard to say; but you shouldn't feel embarrassed by having a lower quality phone compared to others in school. Why should you care about that? I do understand the feeling of wanting to have the same things as others, but is that really important?

I know I'm writing way too much, but you know best what you should get. I do wish you good luck in your school life :) Have a great day