as wrote:
What are the good places where our 14 year old can go off safely with
the instructor for a lesson and learn comfortably?
Do we need to invest in semi-private / private lessons or are group
lessons fine?

We are planning to spend 2-3 days (weekdays, not weekends) on the
slopes and some of the places being considered are Sierra at Tahoe and
Heavenly (expensive?);

No point in Heavenly for a beginner. Someone suggest Diamond Peak, which
was a good suggestion. Stay in North shore, or even in Reno for Mount Rose.

Some places have good deals for first time skiers, for lesson/rental/lift.

Check out REI for discount lift tickets for Mount Rose as well. At 14,
your kid is past the el-cheapo ticket prices. Bear Valley, Kirkwood, Mt.
Rose, Northstar, Sierra at Tahoe, Squaw Valley and Sugar Bowl have REI
discounts at the Bay Area stores.