I'm sorry, but I'm going to have to vehemently disagree with some of the answers so far that want to make spurious claims that "left-handed people have an advantage when playing right-handed guitar/ukelele." Such claims are usually promoted by right-handers who have little experience with left-handed musicians.
If you think about it just a bit, you'll realize that such a claim is patently absurd. If it really were easier (for most people) to play a guitar with the dominant hand fretting and the other hand strumming, and given that the vast majority of people historically were either right-handed or forced to learn to do things right-handed, why don't most people play guitars facing the other way? The answer is that picking is ultimately easier with the dominant hand. The dominant hand controls the fundamental rhythm, as well as a lot of expression and sound of the instrument, just as on violin the dominant hand is used to bow and control the sound production. The more advanced one's picking becomes, the more it becomes clear where the greater dexterity is needed. And we need only look at a list of professional musicians who prefer to play "left-handed" to realize that maybe there is something more natural about that position for left-handed individuals.
(And if anyone could point me to a similar list of right-handed guitarists who voluntarily chose to learn and play in a "left-handed" position, then I'll believe the claims here about it being "easier" for left-handed folks to play a right-handed instrument. But I sincerely doubt there would be many people on such a list.)
In any case, to the OP's questions: yes, it is possible for left-handed musicians to learn to play right-handed, and many have great success. But there is something more natural for most left-handers to play a "left-handed" instrument, so if they begin with a standard instrument, there will likely be a stage where progress slows down or the kid wants to switch to an instrument/position that feels more suitable. (Hence so many examples of professional guitarists who play a right-handed instrument flipped around with the high string on top: the position just feels so much better that they're willing to relearn or reorient the whole conception of note placement on the instrument.)
QUESTIONS: - any readers experienced the same (either themselves or their kids) learning guitar, bass, ukulele? - am I right in thinking that left handers should be thought to their natural ability and not be forced to be right handed?
I don't have personal experience with this as I'm right-handed, and I haven't had to deal with left-handed children. But I have heard anecdotes from left-handed friends.
And here I do have to agree with some of the previous answers in that schools have limited resources. Your situation may vary, but I don't know that trying to force a teacher who clearly doesn't even understand left-handed playing to teach your kid that way is a productive use of your time, your kid's time, or the teacher's time. It's a judgment call. In that situation, I might consider finding a private teacher who had more experience with left-handed players. As long as the school teacher doesn't find it objectionable to deal with the left-handed style in ensembles or whatever, I may not push the issue further.
Or you can just go with the teacher's method and see how your child progresses. Many left-handed musicians do continue to play right-handed (including most violinists, for example, where left-handed instruments are much less common); there may just be some additional challenges along the way.
Questions: - should we insists that my son be taught left hand? - can a natural left hander (forced to learn right hand ukulele) revert back to a left hand style for electric base?
Lots of left-handed players learn to swap sides, as many of them are forced to start out with the right-handed style and make the swap later, so they have some experience. It's certainly not impossible, but it may take a little more time to reorient at first than if a student began with their dominant hand in the standard position at the outset. (Also, bass players often deal with different technique and notation, so a student may just view the whole exercise as a different challenge for the new instrument.)