日本の教育で特徴的なのは、夏に水泳の授業があることですって英語でなんて言うの？

An interesting fact about Japanese schools is that they offer swimming classes in the summer for PE. Japan schools offer swimming for PE in the summer and have their own swimming pools. If you went to school in Japan in the summer you could take swimming in PE at the school. It's cool to go to school in Japan in the summer because we have swimming classes in PE.

"For P.E. In Japan they allow students to take part in swimming classes" 'P.E.' is the abbreviation of the class in school, Physical education. When using 'Allow' this implies permission was needed in order to do something, in this case, take swimming classes. "The Japanese school system is unique for allowing swimming classes in the summer" 'Unique' is an expression that implements that there aren't many things/places like it, this differentiates P.E. between Japan and other countries.

When talking about education in Japan, you want to say that a unique feature of Japanese schools is that they have swimming classes in the summer (for PE). Everyone participates as it's part of school. You use one of the above suggested statements.

"In Japanese schools, the students participate in swimming classes for PE during the summer."- This statement is specific to Japanese schools. It also specifies that it is only during summer time. "PE swimming lessons are held during the summer in Japan."- This statement explains that for PE, swimming lessons are held for Japanese students during the summer. This places more emphasis on the sport rather than it being a unique feature for Japan. "During the summer in Japan, it is compulsory for students to participate in swimming lessons for PE." This gives a statement that the students do not have a choice in whether they want to swim or not. They have to as it is for Physical education.

If in the Japanese school system, every student is required to take swimming classes as part of their physical education, then it is compulsory to do so. The adjective 'compulsory' used in the first statement means 'required by law or rule. It means that it is 'mandatory', every student has to take swimming classes. In other words, every student is compelled to take swimming classes in summer whether he/she likes it or not. The verb to 'compel' used in both the second and third statements means to 'force' someone to do something, in this case, to take swmming classes in summer. So, you may say: In Japanese schools, swimming is a compulsory physical education (PE) sport in summer. or In Japan, every student is compelled to take swimming classes as part of physical education in summer. or The Japanese school system compels every student to take swimming classes as part of their physical education in summer.

"One unique characteristic of Japanese education is the inclusion of compulsory swimming lessons, included in PE classes during summer." This sentence is a formal way of explaining that Japanese schools have swimming lessons. You could alternatively use a shorter version of this sentence: "One unique characteristic of Japanese education is the inclusion of compulsory swimming lessons." "Japanese schools incorporate the unique requirement of including swimming lessons in summer PE classes." This sentence gives a clear implications that the swimming lessons are compulsory and included as part of general Japanese education. "The Japanese school system uniquely incorporates swimming lessons into summer PE classes." This is a more basic expression of briefly explaining the concept included in your question.