Polytechnic

You will have to go through the prospectus of the course provided by the college to know more about the admission procedure, fees, eligibility etc

For enrollment in Aircraft Maintenance Engineering Course at IIAS, Candidate must have passed Higher Secondary Examination (or its equivalent) with Physics, Chemistry and Mathematics (minimum 50% aggregate marks in Physics, Chemistry and Mathematics).
The applicants age at the time of admission in this course should not be less than 17 years. The upper age limit is 24 years and is relaxable if sponsored by Aviation Industry.
Selection Procedure: Selection will be made according to the Merit List on the basis of the performance in the entrance test taken by IIAS authority. Once in a year the test will be taken.

Hi,
The minimum eligibility for BBA is that you should have passed your +2 atleast with 50% and should be under 25 years of age. Various universities gives admission in BBA course on the basis of merit list. Some other college also give admission to students through the entrance tests. The entrance tests is organized at national, state or university level by various authorities. So you should be eligible for BBA based on your Diploma qualification.

You can go through the below mentioned link:
https://www.shiksha.com/engineering/colleges/distance-correspondence-b-tech-colleges-india

For that, you will have to check their eligibility criteria. For this information you can check the website or talk to them in person through helpline contacts on the website. Good luck.

12th does not come under the purview of UGC. So I guess you are safe in that regard.

Admission to the second year of Diploma is not feasible.