Hedrin etc should kill the live lice and the eggs but will not remove the eggs from the hair.
The eggs are glued to the hairs, the unhatched eggs are nearest to the scalp for warmth while incubating, and therefore less noticeable as they are darker than hatched egg-cases and blend in with the hair colour, Hatched eggcases (nits) reflect light and appear lighter and more noticeable as they get further away from the scalp. These will persist in the hair after chemical treatment until combed or picked out.
Most chemical treatments must be used on dry hair.
it is vital that chemical treatments are left on the hair for the length of time recommended on the pack.
If you have been combing using conditioner, then this should be washed off before chemical treatments are used, and the hair dried, as the conditioner, especially silicon containing ones, can prevent the chemical treatments getting to the lice and eggs sufficiently to kill the lice.
Also chlorine can deactivate some chemical lice treatments, so washing hair after swimming, and maybe not swimming for a day or two after treatment, can help to ensure the treatments work.
Itching can continue for a week or two after all the lice treatment.
Chemical treatment should only be used if you find live lice, not ‘just in case’, and you should check everyone in close contact with your child’s head, including grandparents etc if necessary.
Lice are happier in clean hair as it it easier to get the blood meal the female needs to breed. They don’t like coconut, so that’s a good choice for conditioner.
Lice don’t live long off the human body, but if there is a heavy infestation, then changing bed linen and washing at 40 degrees, pref 60, with a few minutes in a tumble dryer, is a good idea, preferably coinciding with treating the lousy heads.
Many people will avoid chemicals and prefer ‘nit-busting’ with nit combing. It can be effective if you are prepared to do this several times a week, for several weeks, and frequent inspections between times. And your child will cooperate.
It is pointless schools sending out ‘Lice Alert’ letters, apart from occasional reminders at the start of school terms, as 20% of school kids will have head lice pretty much all the time, not always the same kids obviously.
Head lice infestations are not limited to primary schools, can happen at secondary also, so don’t think you are safe when your DC’s leave primary ☹️.