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You can reappear for Class 12th in 2019 but the score/ percentage that you will get in 2019 will only be considered for JEE Mains 2019. 2017 & 2018 result will not be considered & no best score will be taken. To get admission in top colleges or to appear in JEE Mains, there is not set percentage that you need to achieve. For JEE Mains, you just need to qualify 12th. For JEE Advance- You should have scored at least 75% aggregate marks in 10+2th standard (or equivalent) board examination. For SC/ ST and PwD candidates, the aggregate marks should be at least 65%.

To know JEE Main Eligibility Criteria 2019,Click below link-

https://engineering.careers360.com/articles/jee-main-eligibility-criteria

All the best.