I would say wake-up time would be aimed at 7am, so how long does the child need to sleep? Most need about 11-12 hours at this age (I am told) so a bedtime of 7-8pm would be about right. Some need more; I know one 5-year-old who needs a 6pm at the latest bedtime to be up for 7.30 the next morning. Some need less; I fear DS is going to be a "less" one. If he needs 10 hours sleep and is fine and healthy and happy on this, I'll do his bedtime routine starting around 8pm and suggest he reads to himself until I come in to say night-night and turn off the light, around 9pm-ish. He can read in the morning if he wakes up early.

Can you let the teacher know this child is up so late? That way the teacher can comment the child seems really tired, could he maybe go to bed a half-hour or so earlier? I know it's hard to get them in bed for 7pm some nights but I think he does need it... that way the mum is aware that his schoolwork and education is suffering but without "come on woman, a decent bedtime would be nice!" said to her face.