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?

Group lessons are just fine for a beginner - weekdays you might even get
group rates with just a couple of kids in the lesson. (Privates work
well when a) you've got money to burn , or b) you're an accomplished
skier who wants to learn a particular advanced skill.)

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?);

Either would be fine; call ahead and see if they have a beginner package
of ticket, lesson, and equipment. Many resorts basically give away the
beginner day anticipating return business. BTW, it's convenient to rent
at the slopes, in case you need to change something; the only difficulty
is crowds, which you won't have on a weekday. It's a pain to rent in the
flatlands and have an equipment problem on the slopes.


No one at our home knows how to ski and we will appreciate any
suggestions that you can pass on;

Don't stress to much - skiing is actually fun.
Stay away from boarding - a first timer will spend most of his first
days banging his butt or head on the hardpack.