Life is what it is. Things don't change unfortunately unless you put an effort into them.
You get asked out a lot, its very much possible you could be seen as intimidatingly attractive. I mean if a person just happens to want to date you, its easy to play that down. Don't base who you talk to on that, base who you talk to on personality, the way they treat you and shared/common interests.
I mean you might've gotten really unlucky, the way to change this luck is when you get to know someone and they're "fun" enough give them a number, heck be bold and tell them you don't want to date but you do want to hang out sometime (if its a guy).
Makes plans.. I mean what is friendship generally if not hanging out? Try to go to the movies, stay at home and play videogames, whatever your interests are. Unfortunately if you want to keep things platonic you have to treat things platonically. Guys (heterosexual) don't call other guys on the phone for an hour just to chat. Too many negative stereotypes associated with that and most guys are immature at a young age . They will talk and listen to a girl sometimes tho, if it comes about that they like you just tell them where you stand. Also relationships like that are more longterm friendships very rare for anyone. Keep in mind misunderstandings come with this sometimes. A relationship between a man and a woman is just being attracted and sharing interests. If you don't want one, stay out of the zone.
I mean what it comes down to it is the whole "starting to be friends" classroom thing, is being a friend imo. An acquaintance that you don't socialize with outside of the school or workplace. Its still being social, its still sharing gossip, its still laughing. If you want to bring it outside of the school, offer to hang out. Also realize why you want the friendship in the firstplace. Are they worth talking to outside of school/work? If not, then you really don't want them as a friend. You're just feeling lonely and as you say.. you're only like that sometimes.
So solutions to that..make yourself active, go jogging (in a safe environment), keep yourself busy with your own interests and do things that are generally fulfilling to you.