How does block-class scheduling work?







By Rajkamal Rao  

Image courtesy: Shutterstock

One of the most difficult things to understand about a high school student's daily routine is the block-class schedule. Teenagers seem to embrace it fine, almost right from the day they are introduced to the idea. It's adults who have a problem in comprehending what at first glance appears to be a highly convoluted system!

School districts engage in block class schedules to maximize resources (class room space, time during a day, the availability of teachers, variety of classes to avoid boredom, and the number of school weeks in a semester). Block scheduling works well whether the school follows a semester system or a trimester system (such as HEB ISD where the year is broken down into three 12-week trimesters).

The easiest way to explain this concept is through an example. Consider Frisco ISD which works on the so-called A-B block schedule.  Here's the schedule for Wakeland High School.

Wakeland High School Day
9:00 am - 4:15 pm

Bell Schedule
1A and 1B periods   9:00 am - 10:30 am
2A and 2B periods   10:35 am - 12:05pm

Tutorial/Advisory    12:10 pm - 12:35 pm everyday

3A and 3B periods   12:40 pm - 2:40 pm
4A and 4B periods   2:45 pm – 4:15 pm

Lunch Times
(A) 12:35 – 1:02
(B)   1:07 – 1:34
(C)   1:39 – 2:06
(D)   2:11 – 2:39

Notice that both A and B Blocks have 4 class periods each, with each period lasting 1½ hours. Each Block-class combination is worth a ½ credit during a semester of 18 weeks.

During week 1 of the Fall semester, a high school student taking her favorite subject would have three A-Block classes (M-W-F) and 2 B-Block classes (Tue, Thu). During week 2, she would have three B-Block classes (M-W-F) and two A-Block classes (Tue, Thu). In effect, each student sits in a 1½-hour class period for five classes over two weeks, amounting to 7½ hours of instruction. This amounts to 7½ x 9  = 67.5 hours of instruction over an entire semester. Why 9? Because there are nine two-week blocks in an 18-week semester.

Suppose a student has the following Fall schedule:
  1. Pre-AP English II (Assume A-Block)
  2. AP Spanish 4
  3. Pre-AP Chemistry
  4. AP Computer Science (Fall)
What this schedule means is that she will sit in Pre-AP English II for three days during week 1 (M-W-F) and two days during week 2 (Tue-Thurs). Over two weeks, she will have sat in 5 English classes, consuming 7½ hours of instruction. Over 18 weeks, this would amount to 45 English classes (9 x 5), 67½ hours of classes. At the end of the Fall semester, she would have earned a ½ credit for Pre-AP English II. To earn the balance of the ½ credit, she would continue taking Pre-AP English in the Spring semester, again on the same A-B Block schedule, for an additional 67½ hours of class time. This amounts to 135 hours of instruction over an entire year, the typical depth of commitment to earn a full high school credit.

So what's the maximum number of credits that a student can earn in a year? That's easy. We know there are 6 instructional hours a day, i.e., 30 hours a week. There are 36 weeks in a school year. This adds up to 36 * 30 = 1,080 hours during a school year. Divide this by 135 hours and you get 8 credits.

What about double-blocked classes? Some subjects, such as AP Computer Science, have too much content to complete in a year through standard single-blocking. By double-blocking this course, the student ends up taking AP Computer Science every day of the week during the entire semester, and thus, the entire year. This amounts to 270 hours of instruction over an entire year. In return, the student earns two high school credits. The below schedule is of one continuing AP Computer Science in the Spring semester.


  1. AP World History
  2. Pre-AP PreCalc
  3. AP Computer Science (Spring)
  4. Debate - 2

[For the record, FISD does not list AP Computer Science as a double-blocked course, but as one that earns 2 credits, so the effect is the same. A better example for double-blocked courses is GT American Studies which integrates two Advanced Placement courses (and like AP Computer Science) is worth 2 credits. GT American Studies combines AP Language and Composition with AP U.S History; therefore, the class is double blocked over the A day/B day time block].

The summary is that students earning a full high school credit in the A-B block scheduling arrangement will take a class 3 days a week, followed by 2 days the next week. They will continue this 2-week schedule for the entire year.


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