Generally you'd need to be identified as having high potential - expressing interest is a good start. However, it's best if you are able to demonstrate leadership capability. Be a level coordinator, head some committees, even proactively suggest and volunteer to drive a project (e.g. remedial programme for your subject, if they didn't do well in prelims)? Or don't create new work to do; ask your RO/HOD what they hope to achieve and see if you are able to help make it come to fruition.

If you've been getting good appraisal (C+ at minimum; A/B is ideal) I imagine it shouldn't be too hard to negotiate some opportunities for you to prove yourself. To take up a formal appointment your CEP needs to have been bumped up from default - it's possible that you have been doing well in your assigned roles, but haven't demonstrated sufficient leadership potential for the school leaders to endorse an increase.

All the best!