At school the patient held the film during periapical radiography and we left the room before taking the exposure. At my current clinic the dentist stays beside the patient and holds the film, so we are inevitably exposed to radiation. How common and harmful is this?

It is very harmful. How many seconds does it take to leave the room? If the patient cannot hold it, I rarely hold it myself, but when patients can hold it one should definitely go outside. In addition, phosphor plates are more convenient in this respect than RVG, so I recommend a phosphor-plate system to those buying a new imaging device for their clinic.