Okay, you say you're punishing him, but what exactly for, and why?
Psychologically a reward system would be better than a punishment system - say for every day he looks after his next tablet he gets +1 point, and 10 minutes of games. Every week if he asks his teacher about his grades (not sure why this should be his focus at 11 years old), he gets +2 points, and 15 minutes of games. Points can translate into things like 'time with mum/dad on my own', 'watch a game of football/soccer/ice hockey', 'have dessert while out for dinner' etc. Having a chart on the wall would provide positive feedback. You could give him negatives for not looking after stuff (eg. putting clothes away is points, doing dishes is points) and give him big bonuses for doing things without being asked, but not directly correlating to points (so not everything has a 'reward cost').
At 11 years old, he has a boy's brain. He's so far off having a man's brain that he may not be 'fully mature' for another 14 years or so (boys mature later than girls)
There's a difference between inattention and irresponsibility. Using tracking tabs like Trackr Bravo (not recommending this, it's just one I'm aware of) to GPS locate his important stuff might be financially benefical. How is he damaging stuff? Does he understand the correlation between item and cost? For instance, my one-year-old understands that when he broke daddy's phone screen, mummy had to go away and work so it could be fixed. He understands that he loses when he messes up - now he tells himself off when he wants to be naughty in any way, which is sweet and hilarious at the same time.
He is doing well at school, but just because I'm...
concerns me a lot. Maybe he's doing well despite feeling like he's not supported? I'd love more expansiveness on how you feel punishment is achieving your aims, and this aspect is something that might need re-working. He might actually like parts of school, but just find he's overwhelmed with the responsibility of having to look after stuff that actually has a value, something that is new to him. Have you asked him if his perception of the situation is the same as yours?
Also, what's the time frame? A month? A year? Since he was 4? Why is he keeping on being given new stuff if he can't look after it?
Also, Otter Boxes are awesome cases a lot of tradespeople use to protect their phones. I'd put his in one, and superglue it shut if he shows any signs of wanting to remove it. It might also be worth buying ruggedised devices.