stupid management

Paid 2nd installment of my school fees. By right the letter sent to me was to require me to pay $2362.50, but the person who attend to me -Cindy told me that they cant find the record of my 1st installment. Then she called our course's planner -Joey and realized that i did pay.

Ok, fine. Since she could link my installment, i went ahead with paying for the 2nd installment. When she passed me the receipt, i saw the amount for 1st installment was $2362.50 where actually i paid $2407.50. I was like 'why did i pay so much? Isn't it divided into 50% - 50%?' I voiced it out and she offered to check for me.

I waited, and waited, and waited. Finally she came to me and said, 'You opted for school insurance which cost you $70. If you add it to $2362.50, it'd be equal to $2407.50.' Problem is, i didn't buy insurance ah, why charge me $70 for no reason!? She called donno who and keep saying is 'Josephine's fault'. What's the point of pinpointing at others when i believe it's not only Josephine doing all this school fees stuff but more like a relay of people doing it.

Immediately i took out my calculator and do the math.
$2362.50 + $70 = $2432.50 It doesn't tally at all!
But if i were to deduct this: $2407.50 - $2362.50, the answer would be $45.

She asked for my contact and said to call me after she investigate the matter. Must have been worried on how to handle me. I waited for 30mins and she finally called.

Cindy: The amount is correct.
Me: So which amount is correct?
Cindy: $2407.50 is correct.
Me: Shouldn't the school fees be split equally into 50 - 50?
Cindy: Ya. Actually it's you who made us realized that we've make a mistake. It's the 5% and 7% GST thing which causes all this error. So that means you've paid $45 short.

WTF! Pushing the blame again. The school didn't calculate properly, is the school's fault. Not that Josephine or GST's fault. If it's really Josephine's fault on doing the sums wrongly, then the school had made a greater mistake by approving to ask us students to pay $2362.50 instead of $2407.50! How can a school make such a terrible mistake? Im so unsatisfied with their ability in managing a private school. Disappointed..