The thing is, it's all very well chuntering about mean teachers, but individual teachers don't set the policy. That'll be the Head.
As a classroom teacher, I don't care if kids do detention for being late to school; I do care that they attend my lessons on time, & I'm glad the school policy is strict on this even when it's my kids.
But the way the school behaviour policy re lateness is set & administered is not my job.
If a student arrives late, they are met by senior management on arrival, told off (& obviously it's checked that there isn't genuine mitigating circumstances in the form of a real disaster).
Then they are held in the cafeteria until next bell so they do not disrupt a lesson by arriving late.
Don't blame teachers - we are busy teaching, didn't set the policy, & can't excuse your kid because it was your fault - it's not our call.
Take it up with the SLT, who if they are any good, will say 'sorry - not interested - that's the sanction'.
It's also a bit pointless saying 'well I'm not going to let my child do the detention'.
Again, decent school leadership will just say 'fine - missed detention = a day in isolation, then'.
If the SLT are not taking a reasonably firm line, you are likely to have bigger problems with real behaviour issues not being tackled because that horrible bully who is picking on your child? His/her mum definitely doesn't approve of school sanctions, & that spineless SLT are giving into them too.