Why do you think the book is called The Wednesday Wars? What wars are happening in this book?

It is an interesting question that you are asking.  Is the title The Wednesday Wars, plural, because there are multiple wars happening on Wednesdays?  Or is it The Wednesday Wars because each new Wednesday is a new war between Holling and Mrs. Baker?  

I'm inclined to think that the plural is in the title because in the beginning of the book Holling thinks that each Wednesday with Mrs. Baker is a new war to be fought.  Mrs. Baker first attempted to have Holling repeat a math class during their study time.  When that failed, Mrs. Baker forced Holling to do menial labor like cleaning chalk erasers.  The entire time Holling believes that Mrs. Baker hates his guts.  Once October hits, Holling believes that Mrs. Baker has brought an entirely new level of warfare to him when she announces that they will be reading Shakespeare together.  

Reading Shakespeare.  Of all the strategies Mrs. Baker could come up with, this must be the worst.  Teachers bring up Shakespeare only to bore students to death.  And I was going to be bored to death for eight months.  No human being could stand it.  

The second part of your question asks about what wars were happening in the book.  There is the "war" happening between Mrs. Baker and Holling, which turns out to not be a war by the end of the book.  There is a war between Heather Hoodhood and her parents.  There is a war between two architectural firms.  Those are Hoodhood and Associates and Kowalski and Associates.  Lastly there is a very real war happening in the world during this time too.  That is the Vietnam War.