While I am, of course, for the promotion of student safety, there are far more pressing concerns than the Giant Squid, who has until this point never hurt anyone. And there have been enough students in the lake, that if it wanted to have hurt them, it easily could have.
And I have to wonder what creatures they are refering to when they say 'invasions'. While indeed there was a troll in our first year, the werewolf in question worked for the school, and was an excellent professor at that. What other 'creatures' might they be classifying as 'invasions'? Nifflers? Dragons? Foreign students? Unruly students?
Muggleborn students? Really, the whole thing is absurd. As a school for teenagers with magical abilities, could there ever have been any doubt that Hogwarts would be dangerous, creatures or not?