With the exception of the Iphone SE which Target is selling unlocked for I believe 129--which BT is the same phone as the 6S with a smaller screen-- the only place you can buy an unlocked Iphone with no carrier is the apple store or of course used or some sellers on EBay... All other places will require you to sign up with a carrier even paying the full price of course the phone can than be unlocked after you are with the carrier for whatever term they require. The only carrier that sells unlocked full price Iphone's is Verizon but again you have to get a plan to buy it...
Most kids at that age do have phones ranging from Iphone 6-10 few will have a 5S or SE and even less other brands.... It stinks...but like whatever was in when you went to school and everyone wanted. The big thing is Fortnite you need an SE or 7 or higher to play otherwise it will run slooooow.
My son in 7th grade has a 7 that I bought from the apple store when it was still 550... I just bought an XR from the apple store... I also have a great phone plan for 25.00 a month all in for each phone through the back end of a major carrier... Also when you buy a phone through apple you can go to any carrier as they are not locked to any nor are they GSM or CDMA specific they will also work in other countries if you travel (you will pay for the service). Some carriers will sell you a GSM or CDMA only phone check the models before you lock into a carrier if you go that route.... In general Sprint and Verizon both have the universal Iphone model where as ATT and T-Mobile are in most cases GSM only phones or have less of the CDMA bands and will not work well or at all with a CDMA carrier.
For the record T-Mobile owns Metro PCS, Sprint owns Virgin Mobile and Boost Mobile, ATT owns Cricket if you watch the deals as they do compete you can get sometime unlimited data, calls and texts for 25 a month all in when you switch a few lines and you can add new ones. Boost usually has deals on Iphones but be aware they are 1 year to unlock and may not work on GSM carriers as they do discount the phones but make sure Sprint works well in your area.
Most important is the case for budget friendly check Supcase on
Amazon and the is another brand called BBalsm I believe they both have screen covers and large corners on the cases and work very well for drops and your kids WILL drop the phones...
If your kids are OK with an SE that is your best choice... cheap and still an Iphone and a perfectly good functioning one at that... if not you may have to come to the unfortunate conclusion that you will pay big bucks for the phone or the plan to get the phone discounted... no Major carrier gives you a discounted Iphone unless you get the higher end plan... or you can buy them for a reputable place refurbished from EBay or like.