What are the attitudes of the schools to the experience of physical pain during sitting meditation?
Pain is obviously related to physical posture; obviously related to the mobility of an individual's body &amp; joints. Therefore, some individuals without any samadhi (meditative development) can sit for long periods while other individuals (who might have better samadhi) cannot sit as long without experiencing pain. 

For example, in the West, are often found slender naturally flexible women leading hatha yoga classes, who display no evidence of spiritual development (e.g., they are emotionally unbalanced, addicted to cigarettes, etc).

Imo, the above shows there is no relevance or correlation between physical posture/pain and meditative development. If it was otherwise, a naturally physically flexible person would either be advantaged or disadvantaged compared to another person. 

When samadhi (concentration) is genuinely developed (absorption into breathing, jhana, etc), awareness of pain disappears from consciousness, even though the legs themselves may become very stiff &amp; very painful. A samadhi practitioner may sit for a few hours without consciously experiencing pain then, after emerging from samadhi, will struggle to stand up &amp; walk. 

This is why I am not aware of any instructions in the Pali teachings about meditation &amp; physical pain. In fact, the very essence of the Middle-Way is the development of jhana (with rapture, happiness &amp; equanimity) which is contrary to those Theravada schools (Goenka; Mahasi, etc) that teach meditation on pain. 

Since the ultimate purpose of Buddhist meditation is to develop a bliss (samatha) that liberates an individual from reliance on sensuality &amp; then insight (vipassana) that liberates a individual from attachment, self-view &amp; suffering, it is obvious meditation on pain is not directly relevant to the Buddhist path.

When I was young, I practised meditation very intensively and developed some capacity to be non-attached. Then at a later time, my body suffered an injury &amp; a lengthy rehabilitation that was very painful at times. I required no additional training in non-attachment to deal with my injury &amp; rehabilitation. The non-attachment developed in my painless meditation, when I was younger, was more than sufficient. 

It may sound like a paradox but non-attachment &amp; dispassion towards rapture (piti) is said by some to be much more effective at dealing with pain that direct meditation on pain itself. This is probably why the Buddha encouraged the development of non-attachment &amp; egolessness (rather than meditation on pain itself).