Welcome to school nursing! You'll be fine! This is a healthy population for the most part. You just need to hold down the fort and get them all home alive and you've done your job. You'll probably see things like kids who complain of not feeling well (everything from migraines, cramps, stomachache etc) to injuries from sports or fights or even that may be possibly be high or using something which your background may help with that!
When they walk through the door ask what's wrong and do a quick assessment. Do they LOOK OK, color, guarding any pain, alert, sluggish, etc. This age kids are good fakers. Check temp, ask how long symptoms have been going on, when they started, what brought them on etc. If kids are injured check that out or check for signs of concussion. Always err on the side of caution with head injuries. High school kids can text or call their parents on their cell in front of you and you can get verbal permission from the parent for things like letting them go home by themselves if they can walk or drive and don't have a head injury or seem impaired or too sick to.
If you think they're probably fine (no fever, no puking, acting fine, ate lunch, etc) send them back. I always say well no fever so try to hang in there, try to eat lunch, try to wait a while and see how you do...etc. If you truly aren't sure on what to do because they insist they can't make it just call the parent and leave it up to them. Even if you think they're perfectly fine and they start arguing then don't get into a power struggle. Just call the parent (or have them text or call) and say you're able to go home if mom allows but I'm not excusing it based on the assessment. Let it be a parent note.
If they got hurt and insist they can't move it, walk on it, or whatever call parent to pick them up and have checked out by a Dr. We don't have X-ray vision after all. All head injuries call parent and observe child. There should be some type of protocol to follow for that and all visits.
Call EMS if a kid can't breathe, seizures lasting longer than 5 min (or whatever Dr put unless first seizure ever better call), passed out and turning blue, anaphylactic reaction, or possible broken bone or dislocated knee or something and they can't walk so you may as well call EMS so they can transport them to the ER since you and parent can't carry them. Things like that. Trust your gut. Teachers/staff like to think they know best but they don't.
This is Summer school so maybe only half a day? A lot more laid back at least. You may have some routine daily meds or blood sugar checks and some emergency meds just in case. If you don't know something ask your boss, the secretary, principal, etc. Don't be afraid to ask. We create these scenarios in our heads of the worst of the worst but thankfully most schools there will always be people willing to jump in and help in some way in an emergency and those truly don't happen all the time!
Find you policies and protocols and go from there. Best wishes to you!