Hi

The rules for getting admission in CBSE are as follows

Admission to Class XII :-

(i) No admission shall be taken in Class XII directly. Provided further that admission to Class XII in a school shall be open only to such a student who:

(a) has completed a regular course of study for class XI; and

(b) has passed Class XI examination from a school affiliated to this Board.

(ii) A student who has completed a regular course of study for Class XI and has passed Class XI Examination from an institution recognised by / affiliated to any recognised Board in India, other than this Board, can be admitted to a school only on the transfer of the parent(s) or shifting of their families from one place to another, after procuring from the student the marksheet and the Transfer Certificate duly countersigned by the Educational Authorities of the Board concerned. In case of such admissions the schools would obtain post facto approval of the Board within one month of the admission of the student.

7.6 Notwithstanding any thing contained in paras 1 to 5 of this Byelaws, the admission of students passing qualifying examination from an examining body outside India shall be regulated according to the provisions contained in byelaw 6.2 of this chapter; provided that the condition of completing regular course of study for class IX and XI is satisfied in cases of admission to Class X and
Class XII respectively.

For more on CBSE rules please refer the link below

https://www.cbse.gov.in/cbsenew/exam_faq.html

Hope this helps!