Hi Tess,
If it was me, I definitely would NOT want to be stuck in school for 6 to 7 hours a day, just 10 days after major knee surgery. It could certainly be done (and I'm sure it has), but it's going to be really uncomfortable for him and not great for his recovery. Missing a week or two from school IMHO is not that dire in the grand scheme of things. It's more important to have a healthy functioning knee for the rest of his life and those first few weeks after surgery are critical to regaining full leg extension and knee functionality.
If it was me (or my kid), I would see how he's feeling, and go from there... but more than likely I would plan to keep him out at least that first week back of school until Jan. 8th (but maybe another week after that), and let him recover. See if you can get his studies/ homework sent home, so he can keep on top of things while recovering at home.
I would schedule to have him in PT right away (I was in PT 2 days after my surgery), so they can get him walking with a sports brace as soon as possible (so he doesn't have to get around school using crutches). But I'm guessing, getting into PT that soon may be difficult since his surgery is right before Christmas. In any case, I would get the PT appointment scheduled now (or have your surgeons office setup the appointment if they are connected to the PT), because appointments are usually booked up at least two weeks out at PT offices. I got my sports brace at week 3.. which was when I started to feel more normal, and could move around without crutches.
But you never know, he may get bored or feel well enough and want to go back to school... Since he's younger, he may recover quicker.
Keep in mind, ACL surgery is really draining, both physically and mentally. Just getting ready for school in the morning will be a real chore if he goes back that soon. Concentration levels were lacking for me in the first month or two, due to a throbbing pain I felt in my knee. So just keep in mind, recovery / pain / etc. can be different for everyone, so it's hard to say what will be best for him. If he does go back to school early, make sure he elevates his leg on a chair (or desk) out in front of him, because sitting in a chair for any length of time with the knee bent is bad for swelling and recovery.
Yeah, if you can I would try to get an ice machine... from what I've heard they are nice to have to help dull the pain, but aren't totally necessary. I just used ice packs after my surgery, and can't say they worked all that well due to the thick bandages around my knee after ACL surgery. From what I remember, it was a bit hard for the ice packs to keep the sides of my knee cold.
At any rate, wishing your son a speedy recovery!