Could DSPS symptoms be a deliberate behaviour?

Yes. Some people choose to sleep at late times when they don't have to get up early, e.g. when they are on vacations or when they work in the evening. If they can adjust to an earlier sleep/wake schedule when they need to, they do not have DSPS. Other individuals, particularly adolescents, choose to sleep late in order to avoid school or because they enjoy late-night partying or television. Superficially, school-refusal and DSPS can look the same. Even people with true DSPS often have problems in school, because their chronic sleep deprivation makes it difficult for them to arrive at class on time and then stay awake and concentrate. Closer examination reveals the following differences:

Desired Late Sleep Phase DSPS
Can change to earlier sleeping and waking times when not in school.Late sleeping continues throughout the year, and often drifts to even later times during vacations.
Does not co-operate with treatment.Usually enthusiastic about beginning treatment.
Nearly always clinically depressed.May be clinically depressed.
Unhappiness about attending school usually begins long before the pattern of sleeping late.Absenteeism and other school problems develop, or significantly worsen, after the late sleeping pattern takes hold.

Adolescents who sleep late to avoid school do not have a sleep disorder - but they do have a significant problem which they can't resolve alone. Psychotherapy to treat the depression, and perhaps a change of school or school program, should be investigated.