How common are sleep apnea school problems?

Fairly. Snoring used to just be "a thing" but we're realizing it's a bigger deal. When kids are having restless sleep, mouth breathing, bad snoring, and waking at night, it can affect deep and rem sleep. Not achieving these can affect concentration, memory and alertness during the day.
Sleepiness. If a child has proven sleep apnea he/she will have almost irresistible sleepiness in the day and might affect school behavior.

Related Questions

How common is obstructive sleep apnea is grade-school children?

5% or more. Estimates as high as 5% of all school-age children have sleep apnea. This number certainly will go higher given the unfortunate epidemic of childhood obesity. Obstructive sleep apnea can cause all sorts of problems and children with it are frequently mistaken for having attention deficit disorder. Read more...
Sleep apnea. This can be a very common problem- do they grind their teeth, or are there any behavior problems. Have they had their tonsils and adenoids removed? Read more...