Explanation:

- Option 1 (A): "Students are not allowed" is grammatically correct. It correctly sets the context.

- Option 2 (B): "to handle these chemicals" is correct. It properly describes what the students are not allowed to do.

- Option 3 (C): The phrase "unless they are not under" contains a double negative, which makes it incorrect. It should be "unless they are under."

- The corrected sentence should read: "unless they are under the supervision of a teacher."

- This correction ensures the intended meaning that supervision is required.

- Option 4 (D): "the supervision of a teacher" is correct. It clearly states who should supervise.

- Option 5 (No error): The sentence has an error, as noted in option C.

Given the above analysis, Option 3 (C) is where the error lies.