Here are the mistakes identified in each numbered sentence:

1. **Sentence 2**: "We are interested in work together to reach new knowledge."
- Correction: "We are interested in working together to gain new knowledge."

2. **Sentence 3**: "His family was poor, because he couldn’t go to school."
- Correction: "His family was poor because he couldn’t go to school." (Remove the comma before "because.")

3. **Sentence 4**: "Camp is one of my favorite outdoor activities."
- Correction: This sentence is correct. (No change needed.)

4. **Sentence 5**: "I started to like go camping when I was a young boy."
- Correction: "I started to like going camping when I was a young boy."

5. **Sentence 6**: "A notebook isn’t more expensive as a textbook."
- Correction: "A notebook isn’t more expensive than a textbook."

6. **Sentence 7**: "Although my next door neighbor is not very cleverly, he is hard-working."
- Correction: "Although my next door neighbor is not very clever, he is hard-working."

7. **Sentence 8**: "Although my sister was tired, but she helped me with my homework."
- Correction: "Although my sister was tired, she helped me with my homework." (Remove "but.")

8. **Sentence 9**: "You should hurry up, and you’ll be late for work."
- Correction: This sentence is correct. (No change needed.)

9. **Sentence 10**: "We have did this test for thirty minutes."
- Correction: "We have done this test for thirty minutes."

10. **Sentence 11**: "How didn’t the teacher come to class yesterday? – Because she was ill."
- Correction: "How did the teacher not come to class yesterday?" or "Why didn’t the teacher come to class yesterday?"

These corrections should enhance the clarity and grammatical accuracy of each sentence.