The problem: There's zero chemistry
The solution: Be straightforward. Here's what you need to say: ‘I've been feeling lately that you might want something more than friendship with me. I feel kind of awkward not saying anything, so I'm just gonna get it out there: I don't have those feelings for you. Okay, awkwardness over! What were you saying about the anatomy lab?'
The problem: Your friendship is on the line
The solution: Emphasise what's already good. Say something like: ‘I am such a goof at relationships that I don't want to try something different with you and then screw it up. Can we please just be friends?'
The problem: Wrong team
The solution: Clear things up. Whether you're gay, straight, asexual, questioning, trans, or feeling something else entirely, just be honest: ‘I think you're an awesome person, but I'm not ____.' And it's totally fine to ask them to keep this information to themselves.
The problem: ‘Who are you again?'
The solution: Deflect to friendship. Instead of raising your eyebrows and letting that question sink, unspoken, into his desperate soul, try this: ‘I'm so flattered. I'd love to get to know you better, as a friend. Want to join us for a slice after school?'
The problem: You're colleagues
The solution: Draw the line. Drill the fact that this isn't a good plan into your own head, and then drill it into his/ her head by saying this: ‘Oh, I don't date people I work with. Nothing personal.'
The problem: Lots of red flags
The solution: Go with your gut. Whatever it is that makes you wrinkle your nose in distaste, listen to it! To turn him down, a simple ‘no, thanks' and a subject change (e.g. ‘are you going to the lacrosse game this afternoon?') will do nicely.
The problem: You've already got a plus-one
The solution: Don't lead the guy on. Also don't make promises, and certainly don't start dating him without dumping your current guy or girl first. Say: ‘Oh, I'm already seeing someone. Sorry!'