Leading them on?

I've had a few times when I had to say no to guys. It's not that I've had commitment issued, it's just that I'm not ready to date: I don't have time for guys just yet. And before you ask, I am straight, fall head over heels for some boys sometimes (I internalize my emotions, I don't want rumours running around). Anyways, I feel like sometimes I am leading guys on unintentionally... I don't know what to do! I don't want to start dating until I'm mostly finished University. Also, I'm not the most attractive, so please don't think guys are throwing themselves at me, I just want to know what to do the next time it happens? Anyone experienced this before?