I never had a baclofen pump, and it's true that there are risks, and drs often also have a hard time finding the right dosage... but I've heard from others that when it works it works really well and helps reduce spasticity.
I just had hamstring and adductor lengthening this summer (I'm 19). I've also had Achilles lengthening, about ten years ago. As far as the casts, they're only on for a month, but they are full leg casts (ankles to thighs) so it's kind of rough. The dr said I would need 10-12 weeks of therapy afterward...I have had about 6 and have regained most if not all of my strength (I had aqua therapy...if you decide to do the surgery maybe you could try to find that?) The thing is, the dr said the younger kids generally don't work as hard in therapy so it takes longer/is harder for them to recover. You know how your son would work, though

. As far as school, if he has a wheelchair he could use that and go to school as soon as the casts are off, or even while they're on.
It's a tough decision you have to make because either way, you will be doing surgery. IMO the baclofen pump might help more long-term because even after releases, spasticity comes back and the muscles eventually tighten again. But good luck with the second opinions, let us know what you decide!