Notre Dame -- Warned Football Team ... Beware The Power of Social Media

Back before he posed for a photo with a huge porn star, Notre Dame football player Justin Brent was warned by the school about the power of social media ... TMZ Sports has learned.

Pics of the 18-year-old freshman WR taking in a NY Knicks game with 42-year-old "Nailin' Palin" star Lisa Ann surfaced this week -- and immediately went viral.

We spoke to a rep for the school who wouldn't comment specifically about Brent -- but said in general, "We tell our players, understand when you're at Notre Dame the attention is great. You have to be smart off the field. Especially with social media."

We're told university officials are constantly reminding student athletes about social media.

Ya gotta imagine ND isn't thrilled about Brent and Lisa Ann -- considering it's a Catholic university which frowns upon premarital sex.