Players can still pull out, but they can no longer declare to be included in the draft. Currently the 2020 draft is scheduled for June 25th. NCAA players on this list have until June 3rd to withdraw. International players have until June 15.
Players are allowed to hire an agent without losing their college eligibility. Players can return to school if they go undrafted, BUT this rule only applies to players who participate in the NBA draft combine. I am not sure of the current status of the draft combine which is currently scheduled for May 21-24.
I have marked with a * the players who are likely to be drafted, making them unlikely to return to school.
PAC-12 players:
Timmy Allen - Utah
Tyler Bey* - Colorado
CJ Elleby* - Washington State
Both Gach - Utah
Josh Green* - Arizona
Jalen Hill - UCLA
Nico Mannion* - Arizona
Remy Martin - Arizona State
Jaden McDaniels* - Washington
Zeke Nnaji* - Arizona
Onyeka Okongwu* - USC
Chris Smith* - UCLA
Isaiah Stewart* - Washington
Tyrell Terry* - Stanford
Alonzo Verge - ASU
Romello White - Arizona State
McKinley Wright IV - Colorado
Also of note:
Devon Daniels - North Carolina State
Malachi Flynn* - San Diego State
Matt Mitchell - San Diego State
Omer Yurtseven - Georgetown
Hardest hit will be Arizona (3 best players) and Washington (2 best players). Stanford and WSU will both be losing their best player from a roster that is already thin on talent.
NOTE: Edited to add Allen, Hill, Verge, and Yurtseven