If all you can manage is a little erection, that would be nasty.  But in a dancebelt, a little erection wouldn't be too obvious.  A normal-size or a big erection is a good thing, ordinarily.  In a dancebelt, it won't tent out so much, it will go up more.  If you're prone to hard-ons in class, make sure that your tights are pulled up your belly, and that your dancebelt gives you enough support.

It's just a fact of life:  all guys get erections, even boys before puberty.  So if you are going to pursue a career in ballet or jazz, you have to face up to the fact that you are going to have erections while wearing tights and a dancebelt, and that this is going to happen in class and maybe even on stage.

Don't draw attention to yourself by acting embarassed or by looking down at your own crotch or trying to keep your hands in front of your groin.  Most people in class are only looking at themselves.  They won't notice you unless you get weird.

The best thing to do is to pretend it isn't happening.  Stay calm and casual.  Act like it's no big deal.  But if it persists or if it's distracting, try pulling your t-shirt out in class so that it drapes down and gives you cover.