The students who are already enrolled have a high standard.
ご質問ありがとうございます。
「内部生のレベルが高い」は英語で「The students who are already enrolled have a high level」と言えます。
また、「来年新一年生になる内部生のレベルがかなり高いため、(外部からも)レベルの高い子しか受からない。」は英訳すれば、「The students who are already enrolled, whom will be entering 1st grade next year, have a high standard. As a result, it means that probably those with a high level (including students who are currently not enrolled) will only be able to take the test.」になります。
少し長くなりましたが、ご参考になれば幸いです。