There are no examination as organized by UPSC for vacancies for 12th pass candidate. Now if you wish to go for government service examination after your 12th then you can apply for SSC MTS, SSC CHSL examination or you can apply for state PSC organized examination. Most of the examination organized by UPSC is mostly for graduates and PG's.

To know more about SSC MTS visit the link below:

https://competition.careers360.com/articles/ssc-mts

To know more about SSC CHSL visit the link below:

https://competition.careers360.com/exams/ssc-chsl

I hope this answer helps. All the very best for your future endeavors!