Job notice period of 6 months and resignation issues. Difficult wrkng

Hello sir, i work for a coaching class. When joined i was nt given any appointment letter. Verbally we were told that in case u leave this job give 6 mnth notice period or six month salary. Also they took a blank stamp paper of Rs 500 from us on which nothing was written. We ( me and other few employees) bought it and submitted to HR. Later now i do nt find the working conditions good as well as no growth during this year. The work timings have created health issues for me . Also due to notice period of 6 months i have lost another better opportunities with suitable workinh conditions. Now on this basis i want to resign. My question is that. . . May i leave this job giving 30 days notice from my end or will it be a compulsion to serve full notice period? Will i be compelled to pay remaining months salary? As it is a coaching can they claim that their coaching class will suffer if i leave in between?in past also they have threatened many other.