A Harvard grad who was among 50 people busted in the widespread college admissions bribery scam didn’t need an answer key to get the right answers on the SATs or ACTs — because he’s just a “really smart guy,” prosecutors said.

And Mark Riddell was well-positioned to help kids ace their tests, Massachusetts US Attorney Andrew Lelling said Tuesday.

The Palmetto, Fla., resident was director of college entrance exam prep at IMG Academy and graduated from Harvard in 2004. He played tennis for the Division I ivy.

“He didn’t have inside information about the answers, he was just smart enough to get a near-perfect score on demand or to calibrate the score,” Lelling replied when asked whether Riddell had access to an answer key.

Prosecutors accuse Riddell of accepting bribes from mastermind William “Rick” Singer to either secretly take the college entrance exams in a student’s name or replace their answers with the correct ones.

Riddell was paid $10,000 per test from money that flowed from Singer’s sham charity, KWF, which parents paid into, according to court papers.

The duo apparently took care to not make the students’ doctored scores appear too good to be true.

“Singer would discuss with parents what kind of score was impressive, but not too impressive, and then would instruct Riddell to attempt to get that score, and he was just good enough to do it,” Lelling explained.

Riddell, 36, who is facing charges of conspiracy to commit mail fraud and honest services mail fraud and conspiracy to commit money laundering, is expected to appear in Boston federal court on Wednesday.

IMG Academy said it suspended Riddell, according to the Herald-Tribune. His biography was scrubbed from the institute’s website.