Would you think this was reasonable?
For context my DC's school is consulting on going phone free - pupils will have to put their phone into a lockable pouch when they enter school. They will be subject to random bag checks, and if your phone is not in the locked pouch, or if you are caught using it, there will be an automatic 3 day suspension.
Overall, I'm in favour of going phone free, but I think the punishment is too much, especially as it is harsher than that given for fighting/bullying or disrupting lessons.
I also don't think it will prevent cyber bullying, as, as experienced by both my children, this happens in the evening, rather than during the day.
The punishment is ok - your are being unreasonable
The punishment is not ok - you are not being unreasonable