We consider a single-line queuing system with an infinite buffer that receives two Poisson flows of customers with different intensities. Customers of the first type have preemptive priority over customers of the second type. In addition, at the time of the end of servicing, a high-priority customer with some probability can drop all low-priority customers in the queue. Serving both types of customers has an exponential distribution with different parameters. We show expressions for calculating stationary probabilities in this system, the probability of servicing a low-priority customer in terms of the generating function, and a formula for the average number of customers of the second type.