Giving a letter as a secret admirer?

I'm not here to tell you what to do or not, it's just that I don't really see the purpose of it. You say he's leaving and you'll never see or speak to him again, and he won't know the letter is from you. So you won't see his reaction, he won't be able to speak to you about it and so on. And he could also see it as cringe and perhaps creepy. I like Zarek's advice, surely it's better to at least try and say some sort of goodbye and maybe ask to keep in touch via socials or whatever.

It has the potential to be seen as a bit weird and embarrassing. I would see if, even at this last moment, you can get a word and express a wish to keep in touch. With social media no one is far away

but i am actually scared to do so

my crush don't even use snapchat let alone any other social media

Progressing love interests generally involves tackling and overcoming some fear. All you’ll be saying is you’d love to keep in touch, not (if you’re wise) declaring your love. Give it a go!

there is no point in progressing really
as he is going away and we just 14 so
plus he doesn't use social media

Sadly, this will be another unrequited 14 year old love story, then!

Sadly, this will be another unrequited 14 year old love story, then!