Hi, there are two aspects of your stay and your question.
a) The 90/180 Schengen visa rule. The 180 days period is not fixed at some starting date, it is a moving period. Today, you look back 180 days and count how many days you have spent in the Schengen zone. Tomorrow, you look back 180 days and count how many days you have spent in the Schengen zone. The day after tomorrow, you again look back 180 days and count how many days you have spent in the Schengen zone. Therefore, starting your calculation either from Fenruary 18th or April 21st, both calculations would be false.
b) Having tourist visa subsequent to student Schengen visa. In this case, if your tourist visa is subsequent to your student visa, then the days you stayed in the Schengen zone under your studnet visa do count toward the 90-180 rule. See this thread for more details on this topic:
Does duration of stay increase with 2 simultaneous Schengen visas?
http://www.maxi-pedia.com/forum/index.php?topic=1042.0