I can recommend the Alta Ski School. My daughter started there at age 8 when she was Level 4 after skiing in the southeast for several seasons. We arrived in a snowstorm that dumped 14 inches in 24 hours hours. After a day of ski school, she was hunting powder on the sides of groomed blues on a regular basis. She was Level 6/7 after a couple April trips. She's liked all her instructors. We have friends who have two kids (older boy, younger girl) about the same age who only ski a week at Alta during spring break. I think they started going 4-5 years ago. The boy made it to Level 8 last winter and his sister got to Level 7.
We have spent a week at Alta Lodge in late season (the last week or next to last week) the last four seasons. Skied on fresh powder for at least a day every time, including the year that Easter was in late April. We'll be there March 29-April 7.
The lodges in Alta have specials starting April 1. You might call around to see what they have to offer.
If you can ski April 6, that's Alta Demo Day. Free demo tents from 20+ vendors. Great fun!