No there are no barriers, you can appear in exams while u are going to write 12th exams. As 12th exams usually starts in march but application forms are available from January. Now at the time of admission you will need your marksheet to get admitted. As you already have appeared in exams and now u are gonna sit in inprovement so u can surely go for exam. Here you have to make sure you get more than 60% marks in pcmb(if general).

Hello Jennifer, There is no barrier of any kind. It all boils down to you and how prepared you are to give all the exams. Best of luck

Assuming you have passed 12th in 2017 so there will be no issue in NEET, AIIMS and JEE 2018. Also, you can give NEET and Jee main thrice consecutively when you are in 12th class and AIIMS exam you can give till the age of 30.

Also, to get a rough idea about both exam's eligibility criteria you can go through the below articles -

NEET Eligibility Criteria 2017

JEE Main Eligibility Criteria 2017

Hope this helps!!!

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