Assault shouldn't be handled in school. That is a matter for the police.
Old man, a 9 y/o would ring you the fuck out if you gave him your back. Especially one that has been in jujitsu for a couple years. It's not about strength. If he gets his arm right up on your windpipe and ratchets down on it, you have about 20 seconds to get free or you're taking a nap. A permanent nap if he doesn't let go after you've gone unconscious.
"Oh I would just bust out"
Nah, you fucking wouldn't. Idk if you have experienced being chocked from a rear naked, but it's not a pleasant experience. If he gets hooks in and squeezes you better hope your neck is strong for one, and then you have enough understanding of what's even happening to turn into the choke and create space. Your natural instinct is to roll away from the choke but all that does is create more pressure on your windpipe.
Hey now, some people find face sitting to be a very pleasant experience. ---