I know that everything I do can be meaningful. My rabbi says that everything we do has to be an act of preparation for Moshiach. But how can studying for my high school diploma be a preparation for Moshiach?
Dr. Paul C. Rosenbloom was a religious Jew and a professor of mathematics. He once told the Rebbe about a rabbi who had visited his home and expressed disapproval of the bookshelves. "How can you keep secular books together with Torah books on the same shelf?" the rabbi lamented.
The Rebbe replied that to place Torah in one world and science in another borders on having more than one G‑d. Rather, all knowledge should have the same purpose, to further our awareness of the one Creator of All Things.
So, too, your high school studies are a way that you can further your awareness of G‑d. And since Moshiach means a time when G‑dliness will be obvious, furthering your awareness of G‑d and making Him more obvious in your life fits right in to the preparations for Moshiach.
Here are some suggestions on how to find G‑d in your studies:
When you study biology, you can marvel at the complexity, detail and precision of G‑d's creations. How can a person praise G‑d without contemplating the many miracles of life?
When you study history, you can ponder the Divine Providence with which G‑d ran the show at that particular period, and His mysterious reasons behind it.
When you study English, you can think about how G‑d split the world up into many different languages at the time of the Tower of Bavel, and how prior to that there was a unity prevalent among all mankind—which we need to bring back.
These are all just examples. Do the same with every subject—find the point in it that you can use to connect it with something G‑dly. When you open up your books, don't think "I hate this pointless stuff that has no connection to Judaism or anything real," rather think that "I am now going to dig until I find something that is real here, and then I will focus on that and make it the most important aspect of this subject for me."