Hello Aspirant.
B.Sc in Agriculture is a 4 year Undergraduate course. After completing your standard 12th or equivalent education you can get admission into college after appearing an entrance exam. The primary subjects required for eligibility are PCB (Physics, Chemistry, Biology)
The entrance exams are specific to state. Some of the exams are KEAMCET, MHT-CET, KCET, etc.
So yes you can secure admission into B.Sc agriculture after std 12 but you will have to appear for an entrance exam for the course.

Refer to the following link for detailed information:
https://www.careers360.com/courses/bsc-in-agriculture