School not releaving me even after serving notice period, keeping original certificates.

I've worked in a school as a primary facilitator (main teacher) teaching EY3 (UKG) students for a period of 1.5 years. Since past 6 months, I'm struggling with severe health issues due to 9 hours of continuous standing, I've developed 'Plantar Fasciitis' (a condition of severe pain in feet) due to which I'm unable to walk or stand properly. Doctor has advised a complete bed rest and I'm on medication. This was not only affecting my health but also my personal life. Finally, I decided to resign and sent a resignation letter via e-mail and told them that I need to resign on health grounds and will complete my notice period, despite the fact that I was advised bed rest. I provided all the medical prescriptions and medical certificate duly signed by the treating Doctor, but to my surprise the school management did not accept my resignation and asked me to complete the academic year and offered me salary increment to just retain me. I explained them that I'm in extreme pain and cannot continue. However, the management still did not accept my resignation. Now that I have complete my 2 months of notice period, I'm being informed by the HR and management that they are not relieving me. I'm seeking advice here on what can I do as next step here to get a relieving letter, my security money, and the original certificates which they kept during the time of joining.