How would a guy say "No" in a clever way?

“Sorry, I am gay.”

I’m not sure about “clever” but I would say that being honest is the best policy. You certainly don’t want to be hurtful but nor do you want to let her think there’s hope when you don’t share the same feelings.

There is no perfect way to tell someone why you’re not interested in them and I can’t give you a script for this but instead suggest that you think through the reasons and put them into sincere and easily conveyed words in your own mind and then speak them from your heart, all-the-while trying not to break hers. Good luck.

So, you want to toy with her feelings, rather than give her a straight forward answer that might spare her any embarrassment or crush her spirit? I am the wrong guy to ask about that.

Like this and your other questions, you seem to think he should be responding directly to you. But you don’t seem to see his viewpoint at all. Realizing what your beloved is dealing with will help you understand his responses to you.

He is your mother’s student. And that may be (probably is!) critically important to him, so he will not do something that would jeopardize that with your mother.

Realize that if your mother learns he has a relationship with you beyond occasional phone calls might result in him being cut off completely.

Have you talked this over with your mother?

He has been as honest as any man in a similar position would be.

Nyet, stop russian me.

Say it how it is, and sow how you feel about it. If it was me, I’d say no sorry, I’m not interested. And I’m gay. Stop russian me. Lol.

But nah, just say you’re not interested, no need to piss around the bush.

“Could I have your phone number?” This can mean that the guy either (1) likes the lady and wants to call or (2) doesn’t want to know the lady any better, is looking for a gracious and graceful departure, and will never call. If the latter, it really is a kind and non-embarrassing way to say “no.”

If this guy truly likes you he would leave you with no doubt. Trust me, falling in love or even in like is not this complicated. It’s the easiest thing to do.
Think of it like fishing, you gave him some bait, now relax and let him come to you. If he doesn’t nibble in a few days, move on. He’s not your fish.

I think you are saying that the guy wants to let her know that he isn’t interested in that way, but doesn’t want to make her feel embarrassed or stupid. So, he could tell her that she is a wonderful person and a great friend, and that he would just like to stay friends.