Hello Benny!
NO.

If you wish to aaply through SCB channel (iiser aptitude test) you can also apply the next year of givibg boards.

For example, if you want appear for IAT 2019, you can be a student who has passed his/her 12th board exams in 2018 or 2019.
But nothing earlier than that.
IISER states the eligibility criteria for  SCB channel on their official website:
Students who have passed 10+2 level with science stream in 2018 or 2019 are eligible to apply for admissions and appear for IAT 2019.
However, they will be considered for admissions to the degree programmes only if they obtain the cut-off marks specified for their board.
Hope this helps :)
Thank you!