Deontra Q. Gray, 19, pleaded guilty to the lesser charges Wednesday in Montgomery County Circuit Court.
Alante Saunders pleaded guilty to first-degree murder in November, admitting that he pulled the trigger in the April slaying at Betts' Silver Spring home. Betts, 42, was the popular principal at Shaw Middle School in the District.
Two other teenagers, Sharif Lancaster and Joel Johnson, have also pleaded guilty to lesser charges.
The killing — which has been described as a robbery gone bad — took place after the teens set up a sex encounter with Betts.