你的句子是独立主格,其中 school being over是伴随状态,可以改写成when school was over,其实非常简单.又如：
As Mike had fallen ill,I did the work instead of him.你就把从句部分改动一下,去掉as,主语保留,把had变成having.-----------Mike having fallen ill,I did the work instead of him.
你的句子也是如此,把was变成be,再把be换成being.
现在你会了吗?a good day to you.