The kid who seems fine in homeroom might be running on fumes. Hold the door anyway.

People carry things that do not show, and school is an excellent place to look unbothered. The person laughing at the back may have had a bad night, a bad year, or a house nobody knows about. Since you cannot tell from outside, the only workable policy is to treat small kindness as free rather than as something to be earned. Most of it costs you nothing and occasionally lands where it matters enormously.

Do the small thing without needing to know whether it was required. Hold the door, save the seat, send the message. You will rarely find out which one counted.