Yes! I know how u feel.
My lo is coming 19m too. Lo nap 1 time only. Max 1.5hr or 2 hr. So at night can sleep early haha. At this age they can nap once already 🙂
Before start school lo will awake at 9am latest , always sleep at 9-10pm. Once awhile 11pm+ If awake like early 8am, lo sleeps early.
Suggest you can wake lo up earlier. So lo will use to school’s timing.
Eg Wake up 7am, 8-8.30am breakfast,
11+am - 12pm lunch, 1pm nap (milk before nap), 3pm snacks, 6-7pm dinner . 8-9pm sleeping time.
Do you have school time table & food menu? Have a copy of it. So you can practice school’s routine:)