Is the classroom with 20 students the largest? If so, even the 2702 should be fine for this job, but do plan to replace them with 9100 series since they're EoL soon. We haven't had problems in regular classrooms taking online exams, even in rooms that still had 3500s and 2600s. Larger rooms over 150 or so clients and several APs are a different story with different design considerations, for others reading.
If buying new APs, I like to go with the latest generation if only to get more life out of them before they're EoL. Few clients support 6 GHz now (only a Samsung phone, a MacBook Pro, and a Dell Latitude 7430 that I know of), but the APs will be around longer then today's smart phones will. But, 6E APs necessitate 9800 series controllers and DNA licensing (if you don't have those already).
In most cases, I agree with Leo, let the controllers do the work for TPC and DCA. (Again, larger rooms, different story.) 12Mbps minimum rates for both bands worked fine for us, but I'm moving towards 24Mbps minimum. Others go even higher. That should be done in high-density environments where there are multiple APs in a room, anyway.
I would be careful with that 100Mb WAN link. Rough math says you'd be using half of your WAN link just doing exams, not counting non-exam traffic, but not everyone will be using max throughput simultaneously. Maybe users would normally be doing other online activities and the net throughput wouldn't increase over a normal class.
What is your typical throughput during the day, and how much will this exam be adding to it? Do you have graphing software to monitor traffic on the WAN link, the switch uplinks, and the AP downlinks at fairly high frequency to compare a normal class to an exam class? To get this data, and to verify user experience, I recommend doing a practice exam if a teacher is willing.
