So this is it—senior high school at last!
可翻译成：这一天终于到了——终于上高中了！
this is it是习语（有时也可说成that is it），主要有以下三个用法：
用法一：表示期待的事或盼望的事情终于发生或即将发生，根据具体语境来翻译。如：
This is it—the big sale. 快了——就要大甩卖了。
This is it—I’ll have to make my mind up now. 关键时刻到了，现在我得拿定主意。
Well, this is it—the day we've been waiting for. 好啦，这就是我们一直等待的那一天。
This is it, boys, the moment we've been waiting for. 时机到了，小子们，我们一直等的就是这一刻。
注：教材上的这个句子正是此用法。
用法二：表示足够或结束，相当于this is enough or the end。如：
This is it, I'm going. 好了，我就要走了。
I'm afraid that's it—we've lost the match. 我看到此为止了——这场比赛我们已经输了。
As the car skidded out of control, I thought, “This is it. I'm going to die.” 当汽车打滑失去控制时，我想：“完了完了，我死定了。”
用法三：用于确认结果。如：
We've been looking for a house for months and I think this is it. 我们找房子已经找了好几个月了, 我看这所房子就是我们要找的。
If ever there was an avoidable war, this is it. 如果确实存在可以避免的战争，那么这个就是。
This is it, folks—the best record guide in the business. 就是这个，伙计们——业内最好的唱片指南。
用法四：用于确认重点。如：
“You're doing too much.”“Well, this is it. I can't cope with any more work.”“你太劳累了。”“哎，你说到点子上了。我什么事都再也干不了了。”
用法五：表示打定主意。如：
Right. I’m going. This is it. 是的，我准备去。就这样了。
