NIFT

You can take a drop if you can't give time to your study with school but best is to prepare simultaneously. You can make a Time table mentioning time for your subject study.

Yes, getting admission in NIFT is a great opportunity and also, B.FTech is a good start for your career in fashion designing.

It means that you are in waiting category. If there is any seat available after the entire selection process then you can get that seat.

Till the age of 23, you can apply for NIFT examination.

You could join under the Masters programme M.Des and MFM). You will however have to write the entrance test NIFT conducts every year during the month of February. Your UG background remains irrelevant if you can get past the test and have the right creative aptitude. And, Engineers are cool people who are able to understand and grasp anything easily. The salary package has a wide range. Anything between 4 - 9 Lacs p. a. But there are a few tier-1 companies like trident that offer more. I've heard it goes upto 15 Lacs p.a.
Good luck.

Now its too late. The forms were closed last year in the month of November-December.

https://www.quora.com/How-did-99+-percentile-prepare-for-CAT
You can find tips regarding the preparation for CAT. You will have to purchase the model test papers as they aren't available online. You can go to Flipkart or Amazon and buy the complete book.