What I would recommend is to do at least some minimal research on the city and jot down a few points of interesting destinations! Additionally, I wish that I could've done a bit more exploration throughout the city in the first weeks, especially after school. Many times, after school, a group of us would walk to the market or around the school. The school is in an easy walking distance from the main center of the city!
Don't be scared to go exploring at night! Of course, you should never go alone, but with other students/your roommate, but simply walking in the main center of the city at night is wondrous. I definitely recommend this or even going out to eat.
On weekdays, I would start my day by getting up early, around 7 to get ready, then around 8 we would eat breakfast with our host family. By 9, we would be at school in class until about 12. Then we would have the choice to spend the rest of the day freely, walk back home to eat lunch with our host family, or on many days an excursion as a group.
On weekdays, some days after school, we would have a mini dance class or mini-group trip! Usually on Friday, Saturday, and Sunday we had our main excursions. The excursions on either Saturday, but usually Sunday, were day-long excursions! Some of the places we went on these days include Teotitlan del Valle, Hierve el Agua, Monte Albán, Santo Domingo, and many other mini-adventures! Make sure to get plenty of rest before because it can be very tiring.
My biggest fear was definitely understanding and how my Spanish would improve in the three weeks.
I had a lot of nerves leaving my parents, and the long plane rides, but fortunately, I was met up with my roommate in Texas! Other students apart from Sol Abroad were also on the flight, helping to calm my nerves. My nerves weren't completely settled until after the first week, but there isn't much to do about it.
After a week, it is easy to get comfortable, and the nerves wear off. I'd also been a bit nervous about my roommate, but I love her so much - we still have kept in touch! For my fear of understanding, it was quite challenging for me at first, but all you can possibly do is try your best!
It is completely natural to be nervous about the trip as well as the language aspect. Throughout the trip, I was nervous to voice my Spanish, but what I've learned and now regret is speaking as much Spanish as possible!
Take advantage of the trip! Don't be too shy - NO ONE is judging!
Let me say, this trip was one of the BEST things that have happened in my life! There isn't a day that passes that I don't think about the experience and the people apart of it.
Go to Mexico ready to go, get excited! Mexico is absolutely incredible: phenomenal food, kind, welcoming people, beautiful markets, streets, and shops! Don't get too caught up in thinking about how there is 'only 3 weeks': just get out there, explore, learn, have fun!