Would it be
The entire sixth grade class was checking their lockers.

OR

The entire sixth grade class were checking their lockers.

Sorry I am pretty dumb!

"Was" is correct.

"the entire sixth grade class" is a single object even though it is made up of several parts (students). "Class" is the clue here, it is one object. The trouble is is that you should not be using the word "their" (which denotes more than one). Because your use of "class" is singular, "their" should be replaced by "its": The entire sixth grade class was checking its lockers.

However, this may not be the message you want to convey. Perhaps, "All the students in the sixth grade class were checking their lockers" comes closer to what you mean. Don't be afraid to use an extra word or two to make yourself clear.

Although Minnesota is correct if you follow the old school of grammar, modern usage suggests that either is correct. It depends on whether you mean the group as a whole, or whether you mean the individual members of the group. In speech and informal writing we usually make a subconscious choice of which to use. In formal writing, however, it's probably best to stick with the conservative grammarians and choose 'was'.

I'd like to point out that there is a distinct difference between British and American usage of collective nouns. British practice frequently prefers a plural verb in agreement, where American usage most often prefers the singular. For example, newspapers in the UK typically write, "The cabinet are meeting to discuss...," whereas it is always "The cabinet is meeting..." in America.

In older writing in the UK, it is not at all uncommon to read "The United States are...", although I believe this practice has been given over nowadays in favor of our own "The United States is..."

I hear what you all are saying about the differences between British and American English, but in this sentence, the word "entire" forces the subject to be singular.

"Entire" cannot be used to describe a plural noun. For example, one can't say "the entire apples" or "the entire classes." One cannot even use it to describe uncountable nouns, such as "the entire milk." Therefore, "class" in this sentence is clearly singular.

The correct answer here is: "The entire sixth grade class was checking their lockers."

I agree entirely with Sarai's claim about the noun phrase "the entire sixth grade class." Here, the word "entire" imposes a collective (singular) rather than a distributive (plural) reading on the noun class, so the correct verb form should be singular, namely was.

But her further claim that "entire" can't modify plural nouns isn't right. There are entire libraries full of counterexamples. Entire countries were depopulated in past wars. Entire armies couldn't put Humpty together again. It is best not to use entire apples when baking apple pies. And so on.

It is the co-occurrence of the noun "class" in the singular form with the modifier "entire" that forces the collective, as opposed to the distributive, reading.

You're absolutely right!

Thanks for correcting me. Interestingly, we can say "Entire classes are..." and in British English people might say, "The class are..." but I don't believe one can ever say, "The entire class are..." Perhaps it is the combination of the words "the" and "entire" that forces singularity upon the word "class?" What do you think?

As Mutt says to Jute, quite agreem. The definite article can't co-occur with a singular noun and adjectives like entire or whole.

I thank you for pointing out the way "entire" works above. I had completely overlooked that in my first reply.

Was. It's a funny word when you look at it by itself, isn't it. But anyway, "was" is correct. "Class" is singulas and so is "was." "They" is plural and then you would use "were," etc.