To me the situation is really frustrating. In a lot of my 4 hour classes I start off slow, I really don't know WHY but then the SECOND TEST IS THE WORST OF THEM ALL, THE THING IS THE FRIGGIN DEVIL! I always do bad on the second test and the third test is my best, usually a very high B or a very high A. I end up pulling out of the class with a B or C which is really frustrating to me.
One of the biggest things is using all your resources, and like so many people have said, time management. I have a bachelors in History, a non-science masters, and have gone from Bs and Cs as a Freshman undergrad to all As in both 400 level history courses, language courses, basic MCAT sciences and advanced sciences.
For example, in math and science you have to work problems. If your teacher gives you 80 suggested problems, do them all and if you are uncomfortable with something ask for help or try more.
Also, read before class. Use class and problems as reinforcement. DO NOT read the whole chapter in one sitting. Read 5-10 pages a day every day and reinforce this with relevant problems. You need to not only do the problems, but fully understand the material. Study alone without distractions in a quiet room. Turn off Facebook, Netflix, podcasts, etc.
If you need to, see the teacher, but working problems and focusing on what the teacher emphasizes always works. Teachers also often post pages with bullet points of what topics will be covered, look at these and know them. Review in class problems, and bring questions to class.
This was the same with science, math, history, languages, etc. If the teacher gives you 100 pages of reading and 100 names to know for the test, read a little every day, make notecards, learn what they tell you to.
However, this all requires you to be interested in your classes, which brings me to attitude. You have to want to be studying these things.
I am not a calculus prodigy, but I went into every quiz, test, final, etc. confident that I was going to succeed. Sure I studied a lot, but it was all about staying positive as well.
Going into a test, or in your case the second test, thinking it is going to be hard or not go well will make it go that way. Attitude really can be everything, but obvously you have to do the work as well. I have seen plenty of smart people who get Bs and Cs because they are not confident and are negative.
My best advice is to stay positive, and heed the recommendations above. Make school your top priority. Say no to going out when your work is not done. You will be happy when you get the grades, and get into med school!