If you have absolute clarity regarding your choice to study basic sciences, then you shouldn't bother that much for JEE. If you wish to go for Biology at IISER's you can go for the KVPY examination as well IISER aptitude test. Now if you are wiling to go for IISc, then your struggle for JEE ADVANCED isn' completely justified as you can also get into IISc through KVPY but the preparation of JEE ADVANCED, gives you a thrust towards success but its not mandatory, prepare well for KVPY & you can achieve your dreams. Its not like that, you can't crack JEE ADVANCED while going to schools. There are many rankers who have done regular schooling as well as cracked the examination. So don't pressurize yourself to a specific trends, you can obviously do schooling along with studies for JEE ADVANCED. So if you don't want to miss school, just appear for regular schooling. There are no perfect pathway to crack JEE ADVANCED except your hardwork. So if you can manage your schooling & preparation, there are no issues.

For IISER Admission process visit the link below:

https://engineering.careers360.com/articles/iiser-admission

For IISc Admission procedure, visit the link below:

https://engineering.careers360.com/articles/iisc-bs-admission

Wish you a very productive Evening!