Herard would not have been eligible until midseason after transferring from Mississippi State in January.
Kevin Huerter, a 6-foot-7 sharpshooter, left Maryland after two seasons and is now officially a pro after the Hawks drafted him 19th overall.
Clark leaves Maryland to join a family business after seven years with Maryland. Matt Brady, a two-time Division I head coach, was promoted in his place.
The 6-foot-7 shooter is widely considered a first-round pick after a strong showing at the draft combine.
The big man's return gives the Terrapins a frontcourt anchor heading into next season.
Bruno Fernando is testing the NBA Draft waters and has until May 30 to decide whether he'll return to College Park.
The sophomore, second on the team in scoring last year, is the third Terrapins player this offseason to at least explore his professional options.
Fernando is the third Maryland player in as many years to test the draft waters without an agent.
Jackson will forgo his final two years of eligibility in College Park.
The school is investigating alleged illegal payments to former player Diamond Stone.