To answer, let's talk correlation vs causation.

You mention 35% of your school was Pakistani. So any statistics from this have not only the correlation of Islam but also Pakistan, which is a nation with its own culture. They happen to be Islamic, another correlation. You didn't mention whether your school was an all boy school, but if so, there's another correlation to consider.

As any educated person should know, correlation doesn't mean causation, so why do we choose Islam as the blame, or was it even us who decided that?

Africa is a continent where fgm exists in both Christian and Islamic countries. So there isn't necessarily even a correlation between fgm and Islam nevermind a causal relationship. Note that these are developing countries were talking about - all of them - so there's a finer correlation to work from. And as noted by @bitius already, colonialism has had a major impact on development in these countries.

Islam is extremely sexist racist and homophobic. (from reading and from the experiences my girlfriend told me about).
What is your opinion on this, btw? - How do you reconcile islam with Islamic law in respect of, say womens rights?

You're talking of one woman's experience of Islam in the country she was raised in. I can easily assume that experience to be very real, and I'm unsurprised if she blames Islam for teaching Pakistani people this sexist way of maintaining power over women. All of these underdeveloped countries where women are violently oppressed are struggling for power. They can take power over their women and this is an easily exploited industry. I call it an industry intentionally. It's almost always about money and this is in places where there are not 101 ways to make money, so control of their women is a symptom of their failed economies. It has nothing to do with religion, religion is just used to justify the cruelty. If they didn't have religion, they would find another excuse. The human mind has a magical way of doing that, and is made easier by cultural norms.