hi,

see since there is no age limit to appear in net exam you are eligible to appear in neet even while studying in bsc course. however when you take admission you have to quite your bsc course to pursue medical or other health science courses. since you have valid 12th marksheet and tc there is no problem for admission. also you may show the years as a gap year of studies , if you have passed the bsc course then you may show the pass certificate, however there would be no issue to showing it as gap of year for preparation.