1. Can they already read music?

There's a LOT to learn in percussion and rhythmic notation can get in the way of them reading mallet instruments fluently if they don't work on it consistently - which many won't, because "they play the drums." Already having the basics of that skill helps a ton.

2. Do they have rhythmic awareness?

If they can't keep a beat, they don't need to be a percussionist. While this skill CAN and SHOULD be learnable BY ANYONE, as a twelve year old, it should already be intrinsic to the kid. If they don't have a solid internal sense of rhythm, place them elsewhere.

3. Are they a behavior problem?

Percussionists sit in the back of the classroom furthest away from the teacher (when on the podium), play the "easiest" music (when it comes to most concert literature) and have the most "toys" available to them in class. Do they have the self discipline to not be an issue, or will they turn it into playtime? Check their behavior records from their feeder elementary. If they're consistently getting into trouble, percussion is likely not a good fit for them.

For me, those are my three biggest "must haves" as a percussionist. I'd prefer them to have piano skills since they directly translate to mallets but that's not something that's as common as it was a generation ago.

A less important "4. Can their parents/guardians afford to purchase them the appropriate equipment?" is also something to be considered. Percussion is *expensive* - both up front and ongoing with buying replacement sticks and mallets. You can't *really* make that a requirement, but it's something to consider.