Se refiere al "block scheduling", en que unas clases (materias) se dictan los lunes y miércoles y otras los martes y jueves.

http://encyclopedia.thefreedictionary.com/block scheduling

Este tipo de organización también puede incluir, o se puede aplicar, a las clases de educación física/deportes.

Por ejemplo: lunes: gimnasia; miércoles, fútbol
Otro gurpo: martes, hockey; jueves, gimnasia

Schools began implementing block schedules, sometimes with disregard for the statutory requirements for physical education instructional minutes, in the 1980s. Several types of these block schedules incorporate physical education instruction on a limited basis and do not meet the statutory requirement of 400 minutes every 10 school days. An exception is the daily alternating block schedule, often referred to as the A/B Block Schedule. Although the majority of block schedules are at the secondary level, some middle schools also have gone to this scheduling. Implementation of these schedules caused districts to be out of compliance with the physical education statute, so a waiver was necessary to allow the scheduling. To solve this dilemma, a committee including physical education experts, district staff, State Board of Education (SBE) members and California Department of Education (CDE) staff members came up with a recommendation for a waiver. This group did not feel that they could ask high schools in the state to stop doing block scheduling, so flexibility was sought.
www.cde.ca.gov/re/lr/wr/documents/pepolicy.doc

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Note added at 13 mins (2009-12-22 19:26:39 GMT)
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O "día ALTERNADOS"

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Note added at 14 mins (2009-12-22 19:27:10 GMT)
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bueno, hoy no es mi día para la dactilografía...será que lo fue ayer:

días alternados