JEE

Yes, you can give improvement exam for any subject of 12th class for meeting the JEE Main eligibility criteria of 75% aggregate marks. The CBSE improvement exams form will be available in the month of October every year.

Getting a CSE department seat in sastra is very difficult. It's totally based on how many students applied and attended for counselling. So try upto last counselling or join in any branch get GPA above 9 in first year you will be a given a CSE seat from 2nd year.

You didn't mention about any branch preference. So you can easily get any branch seat in sastra kumbakonam try upto last counselling even of you didn't get in 1st one.

You can appear for stream 2 and there is a chance of getting bioinfo . Try upto 3rd counselling even if you didn't get that in 1st or 2nd. It's totally based on how many candidates applied . There will be only 60 seats for bioinfo

Your expected JEE Main AIR is about 201553. Please write about the home state you belongs. However on the basis of previous year of trends you can get the following colleges.
1. NIT Srinagar
2. NIT Meghalaya
3. TIET Patiala
4. Dr. D. Y Patil Institute of Technology, Pune
5. A. C Patil College of Enginee

Your expected JEE Main AIR is about 190945. The minimum eligibility criteria for getting admission in NITs, IITs and CFTIs is 75% aggregate marks in 12th for general category. While 65% for SC/ST. Hence you are not eligible for getting admission in NIT Jalandhar if belongs to general category.

To be eligible in JEE you must get minimum 75% marks in your board exams or else in top 20 percentile in your board but if you belongs to sc it less than 75%. .