Agreed with Rover - in AmE we would tend to use 'quiet' here.
Where I grew up, Americans are quite averse to using adverbs. If my brother and I were in trouble, my mom would yell at us to 'sit there quiet!' I always took that as 'sit there and be quiet', but as I got older I started realizing it's just because people around there hate adverbs. 'Sit quietly' appeals to the west coast part of me, but 'sit quiet' still feels natural to the midwest part of me, because I think of it not as how you sit there, rather what you're doing while sitting there - being quiet.
(not a teacher, just a language lover)