Hello,
Please any help would be great.
There is this guy and he goes to my school. I have seen him around school and think he's super cute. I added him as a friend on Facebook and he accepted.
Then today I sent him a private message saying this: Hey,I don't really know you but I've seen you around school. You seem like a really cool guy. He was online and din't respond. Did I screw up? Please advice from guy or girl!!
Also age is 15.

No he could have been in the washroom or something ttell him how u feel about him

Time will tell if you did wrong, he may be just trying to think of something cute to say to you. Don't give up, if he doesn't answer send another message.