How should i compliment her?

[Dev]

So, it's my school farewell and there's a girl who i used to like and I want to know how can I give her a compliment.We weren't great friends as we didn't really talk and were in different groups. But we used to look at each other and talked a little on Facebook and liked each others pictures. You can say we are good acquaintances. So, if she looks really gorgeous(as always) and her dress is also attractive...how do I compliment her? What should I say that would make her feel good? I don't want to sound desperate or make her feel that I am trying to hit on her or getting clingy.I only have the best intentions and i only want to compliment her and nothing else. So what should I say so as to make her day and leave an impression? What compliment should i give and how? Please help.

[samanтнa x.]

Wish her good luck, tell her she looks nice