the teacher probably was joking or maybe they said it in a really exasperated, pissed off tone and so it didnt sound jokey? no-one can no bevause they werent there.
ultimately if this was my son i would just make sure he had headbands/hair bobbles in future! and explain to him that perhaps the teacher was joking. i would also apologise to a 7yr old as it is your fault that you didnt send him with appropriate kit.
my ds1 (14) has long hair, really long, much longer than ops son. he always keeps some hair bobbles on his wrist and in his bag so he can tie it up for pe, science, technology and any more practical lessons where he needs to.
its his hair, as long as he washes it and brushes it and ties it back when necessary he can do what he likes with it. thankfully i dont seem to live near any of the idiots on here who think boys shouldnt have long hair! he has never been picked on or called names because of it.
my four boys have all gone through periods of having long or middling length hair and ds2s is now very short. ds3 and ds4s varies,depending on what they want.
they dont look like 'girls' not that there is anythinh wrong in looking feminine anyway.
pathetic judgemental attitudes expressed towards a childs hair on this thread, some peoole are just being nasty for the sake of it.