1985 NCSU Grad said:
https://www.aseaofblue.com/2019/4/5/18296225/nba-draft-2019-rules-agent-deadline
Here is a good explanation of the current rules.
Here's from the ncaa website which also explains other rules that are awaiting the approval by the nba.
"College basketball players who request an Undergraduate Advisory Committee evaluation, participate in the NBA combine and aren't drafted can return to school as long as they notify their athletics director of their intent by 5 p.m. the Monday after the draft.
This change is effective if/when the NBA and NBPA make an expected rule change, which would make undrafted student-athletes who return to college after the draft ineligible for the NBA until the end of the next college basketball season."
http://www.ncaa.org/about/flexibility-going-pro-and-getting-degree
This rule could be huge in the upcoming years