You need to realize that you can waste a lot of time thinking about a crush when you could be saying "hey, let's go out, let's go study, etc." You don't want to play it safe and find out 20 years later that someone had a crush on you and you didn't do anything about it. Don't worry about being rejected. If you have a crush you should ask him or her out today and get it over with. You will either be rejected or not. Either way, you will know what he or she is thinking.