I had crushes on 3 people in October, and I gained some confidence, so I told one of them that I liked him in November. I see him at school every day in a lot of my classes and he’s also at my bus stop. He told me that he’s not ready for a relationship and all that ‘generic’ stuff and kind of started talking to me less after that.
Then, in early December, I found out that one of my other crushes, the one that I liked the most, liked me back. So, now we’ve been boyfriend/girlfriend for more than a month and I haven’t really told the other guy that I have a boyfriend. He’s starting to talk to me more again, and I’m pretty sure he thinks that I still like him. While I don’t hate him, I also don’t want him to say something like he’s ready for a relationship now, and I would have to say no. I’m not the kind of person who likes doing that. What can I say/text to him to make him know that either I have a boyfriend now, or I don’t like him like that anymore without being that direct?
LemonVibe 
You don’t have to say anything really unless he asks you out. Then just say thank you, I’m flattered, but I got a boyfriend in March. I hope we can be friends still.
Then if this relationship doesn’t pan out you can date him if he’s free in the future. It’s not your fault he didn’t move faster then. Don’t be afraid of hurting people’s feelings. A good man will understand. A bad one will throw a hissy fit and you’ll be glad you dodged that bullet.
There are a number of things you can say – or do – to let him know you have a boyfriend. First off, you could just wait until he effectively asks you out, and let him down easy. If you don’t really want to do that, then just be honest with him now, and tell him.
I agree with the comment above – it’s his fault for being fickle. There’s no need for you to feel bad about him liking you.