Today's Ecoview column in the Tuscaloosa News addresses this question:

Q: Can you tell me what a group of squid is called? A squad? A school? I’ve tried to look it up, but I haven’t seen anything definite that says, “A group of squid is called ... ” I would really like to know.

A: As you are no doubt aware, a collective noun is a word that describes a group of things. And collective nouns for animals are an exercise in creativity, from a shrewdness of apes to a pod of baby alligators, a murder of crows to an exaltation of larks and a scold of jays. I believe the most common word to describe a cohesive group of squid is “shoal.”

I've heard both shoal and school tossed around. These two terms are often used interchangeably, though they have distinct meanings:

In biology, any group of fish that stay together for social reasons are said to be shoaling, and if, in addition, the group is swimming in the same direction in a coordinated manner, they are said to be schooling.

Squid can exhibit both types of behavior, so you might see either a shoal or a school.

The TONMO forums have, of course, discussed this question extensively. I think my favorite suggestion was a scrum of squid.