is this a classroom that you run for free or does it actually occupy the time of people that have paid to be there or you are paying while they are there?
There a picture everyone needs to paint between the two and the first option is that everything you do is for free, they are there on their own time, and it's just you trying to be nice to make things better.
The second is that the people and yourself are in a space which may costs $100+ per hour, every hour, that room is in use. Then you want a 'cheap' solution which will be somewhat complex to use and won't give the best results, but it is necessary because the very first really cheap solution isn't proving to be effective. The hangup being the requirement for dual zone audio volume control within the same room.
While I think you should use OWI in-ceiling speakers with audio coming directly off the projector, you can use whatever speakers you want and I would go with a dual zone A/V receiver.
Front and rear speaker controls can be done by using a dual zone A/V receiver connected to both spaces and each zone will have it's own volume. This can be done with receivers which are DECADES old for not a lot of cash. You can hook it up analog from your laptop to the receiver and run the two zones of audio completely separately.
I have used A/V receivers to drive multiple speakers within a commercial environment before and it does work very well, but it was also tied into a control system to make it very easy to use. I'm not sure that what you want to do will be particularly easy to use since you want a bunch of different volume control options.
You also need to understand that any audio that comes out of your computer will always impact all the speakers at the same time.
I will add, that IMO you should have gotten in-ceiling speakers, not on-wall speakers. You want to fill the room with audio the same way that you fill a room with light. Your active speakers will go in the trash can and you will only use the speakers you have purchased.
The Sony receiver you linked will provide audio everywhere, all at once, and you may run HDMI out of your computer, through the receiver, and on to the projector as a solution. But, while you can balance the speakers a bit through trim controls of the receiver, you can't turn up the rear speakers independently easily the way you could with a two-zone receiver.
Something, though quite dated, like this will do both the front and the rears completely independently.
https://www.ebay.com/itm/Yamaha-RX-V2700-7-1-Channel-140-Watt-Receiver-Used-Working-Original-Packaging/273091371213?epid=55028681&hash=item3f9583a0cd:g:nrsAAOSw5WVamK1-
You would hook up the analog stereo output from your laptop to the receiver to get the two zones working properly.
Newer dual zone A/V receivers could handle everything using the HDMI connection out of your laptop.
What is the make/model of the projector you are using?
