Our snare line at my high school plays match, but next year my instructor said that we are switching to traditional. So this summer I want to build up on my left hand. As of now, when I try traditional, I feel like I'm not doing it right, and my stick doesn't stay in the center and moves a bit. Do any of you know a simple exercise or warm-up that could help me keep control of the grip on traditional?

Every instructor will have a different technique and style that they will want you to use to approach playing the drum with traditional grip but one good basic methodology to think about is that your thumb should make straight line to your elbow through all parts of the stroke. That means that the wrist shouldn't break in or out to play. I'm going to assume that you know how to physically hold the stick in your hand but that is a common problem amongst beginners at traditional grip. If you play with the idea in mind that your thumb stays in line with the center of your elbow, that should help with some of your control issues.

What you are describing sounds like basic issues everyone who plays traditional has. The same exercises you do for matched grip work for Traditional grip as well.
I have all of my kids read this for both grips to get a good baseline going. Snare Science Baseline Technique article

8 on a hand. lol

Both of the posters above me pretty much hit the nail on the head, but I do want to contribute one specific part of my own practicing routine that I think is extremely useful when attempting to learn and gain control of the left hand in traditional.

Play 8's with changing dynamics. Yes, this can include bucs, but I mainly mean to play crescendos and decrescendos within your hand. In doing this, as the player, you HAVE to demand full control of the stick in order to practice it properly. If you let the stick do what it wants (for example, letting it bounce around within your fulcrum), then you're not going to achieve the proper heights that you're aiming for within each dynamic change.

This exercise has personally helped me define my left hand fulcrum, and it's always great to have a strong foundation of where your heights are located. I would suggest practicing a pattern like this, all played as double-stops:

3" x 8, Crescendo for 8, 12" x 8, Decrescendo for 8, repeat.

Playing this in double stops will give you the opportunity to define your heights within yourself. If you match your left hand 3" and 12" heights with your right hand, then you've added a sense of clarity that is needed to be a successful rudimentary percussionist. From there, you can help define all of your inner heights as well, and make sure that your left hand's crescendos and decrescendos are even with that of your right hand.

One more thing - Make sure you START OUT SLOWLY. I know it seems ridiculously tedious to practice 8's repeatedly, especially at a slower tempo, but you can ask any successful percussionist on this site, it always comes down to the basics. If you can't play with consistent heights and don't have them defined within yourself as a performer, then you don't stand a chance to proceed very far in your percussive career.

EDIT: WCU managed to post while I was going through one of my long-winded posts that I tend to give on here. I basically just posted in length telling you exactly what he just said, but adding dynamics to it. haha

Hope that helped, and if my "exercise" didn't make sense to you, feel free to let me know, and I'll be more clear.

Happy drumming!
B

I actually had to deal with this and re-train my hand after suffering from a compound fracture of my first metacarpal 6 years ago. 8 on a hand isolating your thumb and index fingers is the best way to train your tenons. Also accent beat 1-8 as well as doing the crechendo,decreschendo to get used to quickly coming from say a 3" to a 6" stroke. Then focus on your regular everyday excersizes that you now use. Another great article snare science has that will be helpful is hyperlinked on the page that warboy posted is Drumming Mechanics - A System of Levers. But start slow and work for consistency first before you worry about speed.

Play legato (full) strokes at slow tempos and be very attentive to the motion of the stick to keep it on that path.

I really like the exercise on Jeff Queen's "Playing With Sticks" DVD where he starts playing legato strokes with just the thumb holding the stick. Then he adds the pinky & ring fingers, then adds the middle finger and index. This might be what PipeStyle meant when he said to isolate the fingers.