Here's a breakdown of the mistakes in the sentences:

1. **Sentence 1**: "Mai went to the Student Support Office **although** she wanted to get some advice."
- Correction: "although" should be changed to "because" for clarity.
- **Corrected**: "Mai went to the Student Support Office **because** she wanted to get some advice."

2. **Sentence 2**: "He works much more hard for his exams than he does for his normal school work."
- Correction: "much more hard" should be "harder."
- **Corrected**: "He works **harder** for his exams than he does for his normal school work."

3. **Sentence 3**: “The doctor suggested that he should drink more water, and takes a lot of rest."
- Correction: "takes" should be "take" to match the subjunctive mood.
- **Corrected**: "The doctor suggested that he should drink more water, and **take** a lot of rest."

4. **Sentence 4**: "He asked me when I had left her party last night."
- Correction: The past perfect "had left" is incorrect in this context; it should be simple past "left."
- **Corrected**: "He asked me when I **left** her party last night."

5. **Sentence 5**: "Are you sure you can get on your new classmates?"
- Correction: "get on" is not commonly used; it's better to say "get along with."
- **Corrected**: "Are you sure you can **get along with** your new classmates?"

In summary, the corrected sentences are:

1. Mai went to the Student Support Office **because** she wanted to get some advice.
2. He works **harder** for his exams than he does for his normal school work.
3. The doctor suggested that he should drink more water, and **take** a lot of rest.
4. He asked me when I **left** her party last night.
5. Are you sure you can **get along with** your new classmates?