Dd is LH. At her first school (reception to year 2), she became progressively quieter, we were told she had mild learning difficulties, she had very little confidence and could barely write.
Unhappy with the school, we moved to another school.
As soon as they found out she was LH, they gave her left handed pencils and letter formation chart. (previous school told us there was no such thing). Within a week, her writing had improved dramatically. Within 6 months, she had caught up with her year group.
Obviously, the change of surroundings will have played a major role in the change in dd, but her teacher tackled her problems brought about by left-handedness immediately, and from that point on, she had no more issues with writing.
I think it's a bit simplistic to say that it's not a disability and with a bit of support in certain areas it will be fine.
Of course it's not a disability, but if the support isn't given in a left handed way (LH pens, letter formation charts etc), some LH children will continue to struggle.
Being LH isn't a "one size fits all" scenario. Some dc will adapt, others will struggle to.
It's a bit unfair on the op, who's dd is struggling, that many of you have come on slating her saying shes making the issue.