Me being a bit nosy, I would accept the request and ask why they want to be my friend now when they made my life a misery at school?
I'd also take the opportunity to have a good nose at their profile and hopefully karma has been at work on their lives - how many years ago are we talking here? I tell my kids especially my eldest who gets a really hard time at school that the best revenge is to love a good life - the youngest says that next time one of them bothers him he's just going to say "when we meet as grown ups, please remember that I will be wanting fries with that!"
Eldest went back to Judo tonight after a break of a year and a half - his suit was too small, so he had to borrow a jacket and belt from the leader. Lo and behold, one of the kids that picks on him from school has started going and was there - a lower rank than my son - my son went for 3 years before he gave up before. Anyhoo, they had no idea that my son had been before and mistook him for a newbie as he didn't have his own jacket and the belt that shows their grade, they got paired up in a match and this kid went in with a cocky smile , my son just looked at him expressionless for about 5 seconds, squared up and threw him right over his shoulder and flat onto his back, put on a hold and end of the round!!! I've never been more proud!!!