At first, it seems like this is a straightforward question. As other Educators have already laid out, there appears to be a series of events that have to take place for Speak to make sense. Without the assault at the party, the cops, Melinda’s truancy, and the attempted assault in the janitor’s closet, Speak would not be what it is. It would be a different book. That’s why you could claim that all of those events are major. The structure of Melinda’s journey relies on them.

However, there might be a subjective way to think about major events in Speak. “Major” might mean something like “moving” or “meaningful.” Meaning is not so objective. In this light, the “major events” are a personal choice and dependent upon the reader.

Maybe a reader is particularly riveted by Melinda’s lab partner. In this case, her relationship with David might constitute a major event. Perhaps another reader is fond of art. In this instance, Melinda’s discovery of Picasso might constitute a major event.

Hypothetically, these events could disappear and the story would still make sense. They might not qualify as objectively central—or major—events. Yet, to reiterate, if you think about major events from a subjective, personal angle, then basically any event in Speak is in play as long as the reader can articulate why it’s major to them.

The first major event happens before the start of the book. At a party held over the summer, Melinda called the cops, which is why her classmates hate her. At this stage, we are not told why she called the police.

Second was the bus ride to school on the first day, when Melinda realizes the extent to which she is an outcast.

Third, Melinda finds a place to hide from everyone who torments her—in a janitor's closet.

Fourth, Melinda attends a school pep rally, at which she is recognized and abused for being the one who called the cops.

Fifth, Melinda is accepted by a clique called "The Marthas", but they make fun of her scabbed lips, causing her to cry.

Sixth, Melinda, who has progressively been talking less and less, discovers the impact of silence when a boy in one of her classes stages a silent protest about xenophobia.

Seventh, when given an opportunity to do an art project with turkey bones, Melinda makes a creepy piece of art which her teacher says represents pain.

Eighth, Melinda passes out when asked to dissect a frog in biology class. She hits her head and has to go to the hospital. Here, she reflects that remaining silent has not helped her.

Ninth, we find out the truth of what happened at that party over the summer. She was raped by a senior student.

Lastly, the same student, Andy Evans, tries to rape her again. This time, other students come to her assistance, and the book ends with Melinda finding her voice again and beginning to enjoy social acceptance.

"Major events" will differ slightly from reader to reader; therefore, the following list is not exhaustive or set in stone.

1. Melinda rides the bus to school. This might not be a major event for a lot of people, but for Melinda, it is. It is her first day of high school, and she quickly finds herself an outcast. Readers have no idea why at this point, but this feeling does become pivotal to the story.
2. Melinda looks at herself in the mirror and doesn't like what she is seeing. Her lips are becoming chewed up, and she can't stop the habit. Melinda puts the mirror away. Her outward appearance is a reflection of what is happening on an inner emotional level.
3. Melinda finds the janitor's closet hiding place.
4. Readers learn that Melinda called the cops on a party, and that is a big reason why her peers hate and abuse her.
5. Melinda sees "IT."
6. Readers learn that IT is Andy Evans, and Melinda makes huge efforts to avoid him. She even stops skipping school in order to avoid detention with him.
7. Readers learn through a flashback why Melinda called the cops on the party. Andy Evans raped her.
8. Melinda tries to warn Rachel about how dangerous Andy is.
9. Andy tries to rape Melinda again. This time it is on school grounds.
10. Melinda defends herself, and the lacrosse team steps in to help. The entire school learns the truth about Andy and Melinda, and the story ends on a hopeful note about Melinda's future.

In questions such as these, I think that the individual has to end up determining what moments they see as significant.  Naturally, there are some basic elements that would be seen as universal, but the idea of constructing "ten" important events is one where nearly anything can be seen as essential.  The development of characters, the introduction of side characters, as well as the growth of emotional dynamics are all aspects which can become seen as "important" events.  For example, the rape of Melinda has to be seen as an important event.  It sets the entire work in motion.  Yet, from this many other elements can be developed.  Some might or might not see Melinda's befriending of Mr. Freeman as an important element.  The shunning of Melinda could be seen by different individuals as an important event.  The future of "it" might be seen as another important event.  Perhaps, this could be seen as another important event.  I think that Melinda's embrace of art and artistic expression is another instant where an important event is evident because it provides an outlet for her ability to "speak," the very basis of the novel.  I think that starting from one point and then constructing different elements that you think are important can help you find the ten events that are being sought.