The rules are these:
1. There ARE a growing number of students choosing... (The real subject is "students", which is plural. "A growing number of" is just another way of saying "more and more"; it serves the function of an indefinite pronoun. Some, more, etc are called indefinite pronouns.)
2. A growing number of students ARE choosing to ... (The subject is "students", which is plural.)
3. The number of students choosing to study abroad HAS been growing rapidly. (The subject is "number", which is singular.)