Plagiarism can be solved with non-AI methods because plagiarism is defined more or less by exact copying. If you mean detection of AI generated essays at school

Plagiarism can be solved with non-AI methods because plagiarism is defined more or less by exact copying. If you mean detection of AI generated essays at schools, then teachers just have to expect better essays and be able to identify them since students now have a new tool. Think of it like grading math homework before and after the invention of calculators. Once students had calculators at home the problems simply got harder and higher level, since there was no reasonable way to enforce "don't use a calculator at home with these artificially simplistic problems."