Exiting and entering the school - FLINT

[Victoria Scott]

No: HPFGUIDX 149234

G.C./Vic:
I was just reading the PS/SS again and it struck me that Charlie's 
friends come and take Norbert off the Astronomy tower, so they fly 
into Hogwarts. Another example is Fred and George exiting the school 
on brooms. How are those two things possible if the enchantments are 
encircling the school, or is it just another flint from JKR?

Vic