I think those are the norm, and you cannot negotiate a higher pay for the classes during winter/summer vacations. Teachers are paid during summer/winter vacations, so schools can demand teachers to work. Salary of public school teacher is based on Table of Standard Salary Rates.
Below is the contract for Taichung city, for example.
Article 5: Leave, Regular Holidays, and Application for Additional Leave
5.1 Party B shall be entitled to a 10-day annual leave with pay, which should be taken within the winter and summer vacations in principle, in addition to national holidays and regular holidays. Party A shall have the right to demand that Party B teach, write lesson plans, participate in student English club activities or teachers’ workshops and so on, which are equal to fifteen (15) classes at least, each week, during the winter and summer vacations.
5.2 With regard to wedding leaves, prenatal leaves, maternity caring leaves, maternity leaves, miscarriage leaves, funeral leaves, official leaves and those are not stipulated in this Contract to be taken by Party B, the relevant rules and regulations adopted by the Local Government and Party A shall govern.
5.3 Party B shall be entitled, within an academic year, to take a personal leave for an aggregate period of no more than five (5) days; both personal leave and sick leave for an aggregate period of no more than fourteen (14) days; and shall be subject to a salary reduction on a daily basis for each day beyond the fifth day of allowable personal leave, or the fourteenth day of allowable personal sick and sick leave. If Party B has any class(es) when he/she takes a personal leave or a sick leave or annual leave, then he/she need to negotiate with local teacher(s) to transfer the class(es), or complement the class(es), or pay the tuition fee(s) for the class(es) to Party A. The amount of the tuition fee is the same as the overtime pay in Item 4.1.2.
- In principle, annual leave should only be taken during winter and/or summer vacation. If Party B has to take annual leave during the semester for extenuating personal or special reasons, the annual leave must be first approved by the Local Government.
5.4 Should Party A require Party B to return to the school during school’s winter or summer vacation to provide assistance in performing teaching and administrative work, Party B shall not reject, but comply therewith.
5.5 Any application for leave or vacation in the above provisions shall be approved/disapproved by Party A. Party B shall comply with Party A’s procedures and apply for leave or vacation in advance. Any application for additional leave or vacation not provided in the above provisions shall also be subject to approval/disapproval of Party A.