Hello I currently do not have access to an XP laptop, but on my current Vista SP2 one, the "Connect to a Network" window dialog looks like this:
That window lists all wireless networks picked-up by my WLAN card; if you hover mouse over one of them, pop-up (aka tooltip) has relevant info in the "Radio Type" row (e.g. 802.11n); I understand that window dialog is pretty similar in Win7 machines, but am unsure about the state of things in Win10...
If you can't find a similar window in your XP laptop (I only ever had a desktop XP system, with only an ethernet LAN card...), perhaps you could inquire the radio type of your school's Wi-Fi by asking an IT staff member or another person already logged-in...
Hope I've helped...
(BTW, I'm in the range of a total of 10 different WLANs, including mine, only one of them is of radio type g, the rest are all n )