Async code hang/deadlock

I have an async job that does a lot of parallel awaiting of tasks. It does things like this:

var pupilsTasks = schools.Select(async school =>
    var q = service.CreatePupilQuery();
    await q.WhereSchoolIs(school);
    var pupils = await q.Execute();
    return pupils;

var pupilLists = await Task.WhenAll(pupilsTasks);

Mostly it’s working just fine, but very occasionally I find that the job has been running for many hours and never completes. (Normally the job should take less than a second to run.)

So my guess is somehow it’s deadlocking.

Is there anything specific I need to watch for when using async/await within a Hangfire job?