I don't believe that your ds isn't allowed to take his bag to lessons.
Let's say it's a small secondary. Even 120 kids going to lockers, rifling through bags, locking them back up etc 5 or 6 times a day is going to be HUGELY disruptive. That in itself is going to take 10 minutes out of EVERY lesson. Add in the likely covid precautions/risk assessment that stipulate gatherings of kids should be avoided where possible, and 120 kids on the corridor isn't going to be compliant. Nah, not buying it.
Secondly, you're telling us that your ds is fully equipped. But also in a previous post, he's lost his pencil case. You've known for MONTHS that he's going back to school on (week commencing) 8th March yet he's lost his pencil case, presumably at home, during lockdown, you've ordered one, and he's still using a scratty ziploc bag. Nope. Supermarkets, Home bargains, B&M, poundshops etc all open near me, and it's been 2 weeks since they've gone back. You need a refund on your order and a trip out to buy a proper pencil case.
Teachers are SO pissed off with shit excuses at the moment. Kids are not allowed to share equipment due to covid (and actually, why should they anyway), teachers cannot afford to give pens away nor should they.
On the plus side, hopefully it won't happen again now you both know that he's going to get a comment for it. He needs 10 pens in his pocket if he looses them, not one. Do him a favour and support him to get it right, and support the teacher to not have to waste time ensuring kids are properly equipped.