I thought you might call friends "Primary" if you mean ones who went to the same primary school. If someone is classifying friends into tertiary, secondary and primary groups, then primary friends would be ones put into the first group. Like Andrew says, "primary friend" is not a well known term, so if you have read this in texts, please shame them Mohammed, as that will make your question far easier to answer with certainty.

We don't use the term primary friend here. It's "best friend". Like "when I see my best friend, I always give him a hug."