WTF is a tennis class?
On a more serious note, your mom hasn't expressly forbidden you from the team. GO for it. C'mon bro, it's now or never. Win your mom over, if you can't, then try use the power of logic- tell her how tennis will help you get into college, keep your mind off of girls blah, blah. If that doesn't work, go for broke. Tell her if she doesn't let you try out you will start smoking drugs, that oughtta catch her attention.
P.S. Don't actually smoke drugs. Just tell her you will.