Since each of the 12 dentists serve at least $2$ students, they serve in total at least $24$ students. That means that we have $5$ students left over to distribute to a dentist. If we want the most possible number of students that can go to a certain dentist, we want to place all of these $5$ students at 1 of the dentists. Adding back the $2$ students that were already distributed to the certain dentist, we get that the greatest number of students that can attend a single dentist is $$2+5=\boxed{7}$$