Hello aspirant,
our information, right after 12th you cannot pursue Bachelor in fashion designing but other options you are left with are:

Certification course
Diploma course

Some of the courses are:
Diploma in Fashion Technology

Diploma in Fashion Design Diploma in Apparel Design Diploma in Jewellery Design Diploma in Fashion Photography
Diploma in Retail Merchandising
Diploma in Leather Design
Diploma in Textile Design
Diploma in Visual Merchandising

CERTIFICATE COURSES
Fashion Design
Jewellery Design
Accessory Desig
Textile Design
Leather Design
Footwear Design

It is suggested to complete your 12th first as you will have various choices open for your admission and interests. Most of the reputed fashion technology and design institutes have 10+2 as the minimum educational qualification required to secure an admission.
Hope you understand
Thank you