If you have changed your Hong Kong identity card, but failed to modify your student identity information, you can try the following steps:

1. Contact the class teacher or the person in charge of your school as soon as possible to explain your situation and ask for specific solutions.

2. Provide your new ID card and relevant supporting materials, such as the certificate of household registration transfer and the certificate of ID card change, for the school to review.

3. If the school needs you to provide other supporting documents, please provide them as soon as possible and cooperate with the school's handling process.

If the school can't handle this matter alone, it can ask the local education department or relevant government departments for help.

Please note that the specific solutions may vary by region and school. It is recommended that you communicate with the school or relevant departments in order to get more accurate and detailed guidance.