Twin XL will limit your selection. And that’s not a bad thing! The LL Bean ones feel good and hold up well. Bonus: they have flannel for winter.
My kid did NOT wash his sheets frequently, so I would also recommend a pattern in a not light color. Won’t make 'em clean, but won’t make it obvious they’re not.
A mattress topper is very nice to have on top of the mattresses schools use.
Over all, keep it to a minimum. Some people like a small mat with edges for wet shoes snd boots. Great idea for a kid who will use it.