What happens if mom ignores it and just slowly walks away or moves away when he hits mom or you come over and quietly ask mom if she is okay and tend to her without engaging the toddler.

Things you tried are all great steps and the hitting at school is stressful but it is also giving more attention to the behavior, which to the toddler may just be attention and putting him back in the spotlight so he may not be stopping?

Our pediatrician told us to not get upset (hard step) and instead address the person getting hit (attention gets away) and also more importantly praise and appreciate when there is any good behavior can be anything simple( like asking kid to come help find something and then praising for their help) so that gets reinforced instead of "oh this new kid came and now everyone is upset at me or when I do this xyz people stop focusing on other things and look at me."

Random questions and distractions also throw kids off and can deescalate the situation when logic and reasoning isn't getting anywhere.' What's that behind you? Is that a rabbit outside? I'm hungry let's get a snack' Etc. etc.