This info from MousePlanet may be helpful to anyone with SWGE reservations thru mid-June, as the Grad Night Effect could impact other days than just opening day -
For 2019, Grad Nite is held on May 10, 11, 17, 18, 21, 22, 28, 29, and 31; June 4, 5, 7, 8, 11, 12, and 14
For 2019, the event includes park-hopper access to both Disneyland and Disney California Adventure parks during regular operating hours, and a private party in Disney California Adventure from 10:00 pm. to 2:00 am. after that park closes to "day guests."
Schools can choose to arrive whenever they wish during their Grad Nite event. Some arrive right at park opening to maximize their time, while others arrive later due to travel time or other activities. With the Mickey and Friends parking structure now closed to oversize vehicles like school buses, look for heavier arrival traffic in the Toy Story parking lot.
Based on reports since the event switched to this format in 2014, the parks grow increasingly crowded as the day goes on and more students arrive. Grad Nites are often a good excuse to leave the theme parks early to enjoy a nice dinner in Downtown Disney.
Beware: Formal Grad Nites are not the only times that the Disneyland Resort is crowded with student groups. The last weeks of school are popular times for class trips, something that Disney actively encourages through special pricing and activities for school groups. Those visits won't appear on any calendar, but are a fact of Disney life during this season, and have already begun making a noticeable impact on park attendance in recent weeks.
With so many schools on year-round calendars now, you should consider the period between March and mid-June as one big, hazy spring-break-into-graduation season. So while the Grad Nite calendar can give you an idea of when the most high school seniors will be on property, there is no way to know when a school district from Central California is sending its ninth graders for their own promotion celebration.