The best thing to do is to provide your child's school (or camp or daycare) with an albuterol rescue inhaler in case he has an attack during the day. Set up a meeting with the school nurse and teachers to make sure they know how and when your child should take the medication. If the school allows it, you can have your child carry the inhaler, but most children under 9 or 10 can't self-medicate, and it's best to let an adult dispense the meds when necessary.
Next, come up with an asthma action plan that details what your child's triggers are, what to do when an attack happens, how much medication to give, any limitations your child may have, times when attacks are more likely (like during allergy season), and when to seek emergency treatment. Share this plan with your child's teachers, school administrators, after-school caregivers and, if necessary, the bus driver.
Ask your child's teacher to alert you (by written note in his backpack) each time your child needs the rescue inhaler so you can continue to track his symptoms. If he needs the rescue inhaler more than twice a week, you should talk to your doctor about putting him on a controller medication to minimize attacks. Recent statistics show that kids with asthma miss 14 million school days a year, and controller meds can help keep your child in the classroom.
And finally, try not to worry too much. We know asthma can be scary (especially when it's very severe) but millions of children with asthma attend school every year, so it's more than likely your child's teachers have seen the condition before and know exactly what to do.