Hello,

it's not common or permitted to join a lower class such as class 10 directly after completing class 12, whether it's in a regular school or a "dummy" school (if by "dummy" school you mean a school specifically for reappearing for exams). Typically, once you've completed a higher level of education, such as class 12, you cannot enroll in a lower level class for the purpose of reappearing for board exams.

Hope this helps you,

Thank you