Usually calling something a meritocracy implies that it also has public bars for promotion that you can constantly self-test against and so iterate toward.
A common example is a marital-arts dojo: every student can take a belt (rank) promotion test at any time, and as many times as they like; and it is crystal-clear what such a test entails (usually, being able to beat someone who is already at the given skill-level, with both participants using a defined subset of the taught skills that the target ranking "expects.") Thus, even without taking the official test, students can simply ask their peers who have reached that skill level to spar with them, and in-so-doing iterate toward being of that skill level themselves.
For a more hypothetical example: a meritocratic public schooling system would be one where there're no "years" of education, but rather "ranks" (or just a big unified "tech tree" of topic-units to study), with tests to attain new ranks/unlock new topic-units; and where each student can take each test an infinite number of times (presumably with procedurally-generated tests that resist answer-memorization.)
In such systems, you (ideally) will first very quickly equilibrate to the rank in the system that your initial level of merit allows you to reach, and then will continue forward through the system at the rate you're willing to grind to increase your merit.