These are really nicely composed. Perhaps the textbook has a 'key' anyway, but I have suggestions:
1. His love for children made him go in for teaching and he never regretted his choice.
[I've seen this 'go in for...' in several textbook exercises. Though, I have never actually heard anyone use this construction! My 'современный' suggestion would be: "made him <em>go into</em> ..." or "led him to go into..."]
4. I'm afraid it will be difficult for pupils to cope with the task that time [Commonly, the construction is '...<em>will</em> be... <em>this</em> time', so perhaps, "the task <em>this</em> time"]
7. Only with honest work you can earn people's respect. Привычным порядком слов являатся несомненно "<em>only with...A can you... B</em>"
8. Not everyone who gets the profession [проклятие артиклей. "<em>a profession.</em>" vs. "<em>the profession of...</em>". Also a strange sentence. Perhaps 'gets into the...'.] of a teacher in Great Britain takes up a job.
10. My friend had considered medicine as a career before he finished school. He has always thought that work is the most important in the world. [Very interesting sentence! I suggest that in speech this would sound normal, if intoned right. However, the '<em>that</em>', in <em>text</em>, is a bit ambiguous. I suggest, "He has always thought of that work as the most important..." because the grammatical alternative would probably be, "He has always thought that that(т.е. medical) work is..."]
11. Love of music and interest in (the) theory of education made me consider the teaching of music as a career / made me consider teaching_music as a career.
12. Often school leavers have difficulties in choosing a profession. In such case teachers and parents can help them to make the right choice of the University.