In most cases the schools specify that the recommendation must be from a teacher who has taught you, preferable one from humanities, one from maths and sciences. You can use your principal’s recommendation as a additional recommendation.

Or if your principal is up for it, he or she can be your school counsellor.

One of the things that Admission Officers like to see in recommendation letters relates to the applicant’s intellectual curiosity, creativity, thirst for knowledge, frequency and value of their contributions to class. These are traits that can only be observed by a subject teacher, not your principal. Having said this, if you’re the Head Boy of your school with several other leadership positions and initiatives that have been showcased outside the classroom, a senior figure like a principal may be able to better attest to these intangible qualities that a subject teacher’s recommendation would not reveal to the same deserving extent.

Just like how you might structure an essay as for the body paragraphs not to overlap but complement and cohesively build upon each other, you should be as holistic in your evaluation of your application.