Rationales behind or rationale behind?

Generally speaking, rationale is used to describe a set of beliefs.

From Merriam-Webster (emphasis mine):

: an explanation of controlling principles of opinion, belief, practice, or phenomena

And from Oxford Dictionaries (emphasis mine):

A set of reasons or a logical basis for a course of action or belief.

Based on that meaning, if you are following a belief system that includes several principles or reasons, you would use the singular rationale or rephrase your sentence:

There is a rationale behind my belief that history classes should be taught.
There are several reasons behind my belief that history classes should be taught.

However, it's possible that you are actually talking about something that is supported by two different rationales:

Let's look at two different rationales for the preservation of life: religion and law.

In this case, it's quite correct to use the plural rationales. But, if that's what you actually mean, you will want to clarify that in the sentence itself, or in a sentence that follows, so it's clear it's not a misuse of the word.

For example:

There are a couple of rationales behind my belief that history classes should be taught. One involves the education system and the other the psychology of group learning.

Without further context, it's not possible to determine which of the two situations applies to your sentence. Depending on what you're trying to describe, either the singular or the plural may be correct.

The "rationale" is the set of principles which underpin some argument. In this respect, using "rationales" as a plural to explain a single item (i.e. the belief mentioned in the example) is incorrect, as the "rationale" (singular) can include multiple "reasons". I would simplify it by using "reasons" in the sentence above, or rewrite it to indicate that the rationale includes multiple reasons, although that sounds a bit forced.