Step 1: Understand the question.
To improve students' achievement levels, what is most necessary?
Step 2: The key idea.
Regular checking of progress lets teachers see how students are doing and give quick help where needed.
Step 3: What continuous evaluation means.
It means testing students again and again through the year using class tests, assignments, projects, observations, and oral checks, not just one final exam.
Step 4: Why it raises achievement.
Frequent feedback finds weak spots early, so they can be fixed before they grow. Students keep improving step by step.
Step 5: Check the other options.
Higher teacher salary, changing the curriculum, or a better building may help in general, but they do not directly track and lift each student's learning.
Step 6: Final answer.
It is necessary to evaluate students' achievement continuously.
\[ \boxed{\text{To evaluate students achievement continuously.}} \]