In my career I am fortunate to have taught some of the very best students in the UK, a few of who have already gone on to far surpass my career achievements. But that's not always the case.
It's not uncommon that a student asks me for an reference - often at the last minute in a terse email - for a position that I know from experience they have little or no hope of getting. I have no wish to discourage student ambition, but this causes me a lot of difficulty. The best students are well aware of their capabilities and potential - but less gifted students often are not. Fueled by rising panic, they apply blindly for incredibly competitive positions their academic records and c.v.'s mean are well beyond their capacity to achieve.
If I have the luxury of sitting down with the student and discussing careers calmly with them (i.e. not the-deadline-is-today-because-I-left-it-too-late), I approach this problem along Plan A (optimistic targets) and Plan B (realistic experience collection) lines. I don't want to discourage any student, but I have real moral dilemmas when second rank students don't have an accurate assessment of their prospects. How should I advise them when I "know" they're being over ambitious? Is is my role to act as gatekeeper? What do you do in such circumstances?