In my attempts to create a reasonable timeline I have ended up making two conclusions. A) The Alfea School Year begins in October and ends in August. B) Gardenia is located in the southern hemisphere. Note: I am unable to conclude one theory that agrees with all information. This theory agrees with all the information except for the Christmas episode in season 5 where it is shown snowing in Gardenia during December.
The End of the School Year
At the beginning of season 3 and at the end of the Winx's second year at Alfea it is announced Stella is having a princess ball. A princess ball is described by Aisha(Layla) as being "a coming out party for royalty" or in other words a coming of age ceremony. Combined with Brandon describing of his present for Stella during the princess ball as being for her birthday, it is safe to conclude that Stella's princess ball occurred close to her birthday, August 18th.
"I've been waiting for you for 4 months."
In the Secret of the Lost Kingdom after arriving on Domino(Sparks), Bloom tells Sky that she has been waiting to hear from him for 4 months. Bloom had just celebrated her birthday which is on December 10th, so she made this comment in December. Therefore, the last time she saw Sky was 4 months prior, in other words August. Furthermore, Bloom had last seen Sky at the Winx's Graduation Party, so the Winx graduated in August.
Gardenia in the Southern Hemisphere
The Book Adaptation of Secret of the Lost Kingdom
I have had the pleasure of having access to the Italian book adaptation of the movie. I specifically translated the section describing Bloom's time in Gardenia and surprisingly it said something along the lines of "spring passed quietly in Gardenia." I initially thought this was an oversight, because the time from August-December if autumn, the exact opposite of spring. Then I had the realization that if Gardenia was in the southern hemisphere, then the season that passed would indeed have been spring.
It is made very clear that in the very first episode, summer had just begun, and that the Afea school year was just beginning. Now, keep in mind we have already concluded that the Alfea school year ends in August. If Gardenia was located in the northern hemisphere, the Alfea school year would only last 3-4 months. Similar with the last piece of evidence, if Gardenia was located in the southern hemisphere, this problem would be corrected.
The Beginning of the Alfea School Year
The Halloween Special
The final part of my theory is fairly straight forward. By the time of the Halloween episode in season 2, the Winx had already started school. Therefore the Alfea school year has to start at least a few weeks before Halloween.
Other Interesting Conclusions
If Alfea's school year is indeed form October-August, then midterms would occur in late February/ early March. In season 1, midterms occur after the Day of the Rose which is Febuary 12th, so it complies with the given information.