Manpower supply - Goods and Services Tax - GST

Taxpayer is firm engaged in the business of Manpower Supply to Hospital, Medical institutes, University and supporting staff to Forest departments. They thought its exempted since providing it to hospital forest department and MGNREGA Scheme. They have not collected tax as well and filed as exempted. Now the order is passed asking to pay tax on that since it does not cover under sl.no 3 notification 12/2017 . Any inputs on this. Thank you