A dummy school is that sort of school which gives admissions to students without any need of attending the school regularly.The students just need to go once in a week 

Many NEET and JEE aspirants nowdays prefer dummy schools rather than the regular ones because they can’t afford to waste their time in those schools which don’t provide good quality education.

Hope u found my answer helful.

ALL the best

Hello suken

If you are preparing for jee than you should take dummey school because you need proper time for the preparation and if you do regular schooling ; it will waste a lots of time ( going coming assignment lectures etc. ) That time you can consume better by self-study.
But it doesn't mean that you do not study and prepare for boards ; before boards exam you should learn the way of writing in boards exam and some topics also which is important for exam ( mainly theory parts ).
And don't take it lightly .
Manage your time and next one year properly.
So if you think you can study and prepare yourself ; than you should take dummey schooling.
Good luck