这个when引导的既不是定语从句，也不是状语从句。这里的 when 是并列连词，相当于and at that time，意思是“这时（突然）”，句意为：在那天上学的倒数第二节的体育课上，我正一边走一边看书，这里我突然看到一个女孩独自一人走着，也在埋头看书。

以下句子也属类似用法：

I was taking a walk, when I came across him. 我在正散步，这时候碰见了他。

She was dancing with Jack, when Tom suddenly cut in. 她正和杰克跳舞，这时汤姆挤进来抢舞。

It was at the tip of my tongue to tell him, when the phone rang. 我差点就要告诉他了，这时突然电话铃响了。

I was driving along, minding my own business, when the police stopped my car. 我一直开车前行，好端端的警察就把我拦了下来。

I was just sitting there, minding my own business, when a man started shouting at me. 我就只是坐在那里，也没招惹谁，这时有个男人朝我大吼起来。