DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) State	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) State School Campus Segment	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Business Activities [Abstract]
Number of schools | School			22
Number of states in which schools operate across the United States | State	14		14
Number of campuses treated as destination schools | Campus			5
Number of reportable segments | Segment			2
Liquidity [Abstract]
Cash, cash equivalents and restricted cash	$ 47,150	$ 27,593	$ 47,150	$ 27,593	$ 38,026	$ 38,644
Cash balance	31,300		31,300		$ 20,800
Line of credit facility, remaining borrowing capacity	21,000		21,000
Income Taxes [Abstract]
Interest and penalties expense	$ 0	$ 0	$ 0	$ 0