Puppy doesn't want to be left at school gates

[Roseberrry]

My puppy is about 14 weeks old and I'd like to be able to walk him on the school run. He hates being left though, just pulls and pulls when I tie him up and ends up really sore and retching when I have done it. He is fine left alone in the house so don't think it's separation anxiety, plus there is a janitor stood near him so he's not alone.
How can I get him used to this?

I take my pup on the school run every morning, then we go on our walk from there.
I don't leave him for a second ... I simply shove the kids through the gate

I would never ever ever leave my dog tied up outside any place under any circumstances.