Consider reserving the lab as early as you can. If you have a guest speaker giving a workshop who needs a projection screen, plan the workshop around available dates FAR ahead of time. If a science fair project is due in late January, it is not too early to block off a reasonable number of days as early as September. Some teachers with set deadlines have reserved the lab for dates in the second semester. In contrast to long range planning, if you view the Lab Calendar web page today and see there is an open period you could use tomorrow, it is not to late to reserve it.
Plans do change, and it is OK to cancel.
Please let the lab staff know you will not be coming to the lab as early as your plans change to open it up to others. The lab staff is generally unable to tell ahead of time how many available computers there will be for each individual class. Teachers should ask the teacher using the lab to see how many students are in that class. Often teachers request only a few computers. The number indicates computers reserved for the period.