I have a fixed aim that I want to do mechanical engineering. So, is diploma better or class 11th then 12th?

You need to meet the eligibility criteria to appear for LPUPET and to get admission in Lovely Professional University.

For girls with a Science background after 12th, high-salary courses include Engineering (CS, AI, ECE), Medicine (MBBS, Nursing), Data Science/IT, Pharmacy, Biotechnology, Psychology and Architecture. Fields like Data Science and AI are also emerging fields for girls after 12th science. 

For admission to RIE courses through the CEE, a minimum of 50% marks in aggregate in your 12th class (or equivalent qualifying examination) is required for general category candidates.

Reserved Categories: A relaxation of 5% is provided, requiring a minimum of 45% aggregate marks for candidates belonging to the SC, ST, and PwD categories. 

Hi...

I need to clarify first: are you asking about admission to Chandigarh University for B.Tech/B.E, or admission to a medical/paramedical course (like B.Sc Nursing, MBBS, BPT, etc.)

The marks required and exam rules differ a lot depending on the stream.

Yes, Yale University accepts international students after 12th CBSE. International students can apply for the Yale University bachelor's courses. Given below are the admission requirements for students applying for the UG courses here - 

• A strong academic record 
• High scores in SAT or ACT (In SAT: 680 to 800 and in ACT: 31 to 36)
• English language proficiency (TOEFL: 100 and IELTS: 7+)
• Essay 
• SOP 
• Teacher's recommendations