SCT Controller voiced concern regarding multiple instances of inadequate command of English by local area pilots; noting a deterioration of safety margins in the system.

Narrative

Multiple instances of inadequate command of the English language by many student pilots based primarily at KSEE airport are contributing to an unacceptable increase in complexity at this sector and others. We are forced to ask the instructors to take over radio communications since we are unable to understand the student's transmissions. This ongoing; several years; situation is causing the degradation of acceptable levels of safety as it requires us as controllers to devote an inordinate amount of time and attention to basic communication. Today's occurrence was made particularly acute since three separate aircraft from two flight schools at KSEE were being provided IFR services during a period of complex traffic at a sector which provides approach services to San Diego Lindbergh airport and three other lesser airports. This situation must be rectified to prevent any further deterioration of safety margins. Recommendation; whatever metrics are used to determine whether a student pilot has an adequate command of the English language sufficient for training in a complex environment may need to be revisited. We have been advised in the past that these students have successfully completed English proficiency exams but from personal experience this is an ongoing issue that must be corrected.