The girls even say they felt they had to withdraw because the team needed points. Sure sounds like bullying to me; from the private school and their teammates. Under the regulations of the organization they play for, the girls have an unquestioned right to participate and obviously had earned their spot on the field. Any pressure on them to give up their rights is absolutely bullying.
You might conceivably say they were bribed, but you don't bully people by giving them the points they want.
The other team was willing to take the consequences of refusing to play and forfeit the game. The girls demanded the game go on, and chose to accommodate the boys by sitting out to gain a further league advantage in the standings. Which they achieved. Everything you described about pressure was put on them by their own team. Criticize them for it.
It was obviously a bit late in the day for the Islamic team to not join the league because its rules allowed exceptional circumstances where girls might play on a boys team. All the boys' school did was admit their oversight, apologize for the consequences and try to withdraw, exactly as the rules prescribe. It's not their fault if those rules don't reward a team that wins by forfeit. When the other team said they wanted the game played, they played, just as the rules and sportsmanship require.
What exactly should have happened there on that day?
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PS: ROPSAA is the rule-making body running sports in which these schools play. They have separate boys and girls leagues for soccer. Of course coaches and school staff should read all the rules and fine-print closely, but in the routine stuff I saw on the web-site about showing up, forfeits and such, there is no mention of girls playing in boys leagues. The Justine Blainey decision made it the law here that a girl has a right to win a place on a boys team and to play in a boys league
if there is no girls team for her, but no one has yet said the ROPSAA rules actually spell that out. Most folks glaze over just reading sports rules, and never ever get to court decisions. Since the Islamic school had played in the boys leagues for years without ever encountering a single girl on the any boys team opposite, I think a little common-sense understanding of their unawareness is appropriate. But as no one is actually arguing what the black-letter rules say or don't say about girls in boys leagues, what the Islamic school should have known but did not is all spilled milk.
What exactly should have happened there on that day?