Yomiuri (4/10/2011) reports that the Ministry of Education (full name is ridiculously long, and it is "Ministry of Education, Culture, Sports, Science and Technology) is about to set the limit on radiation level in school yards in kindergartens and elementary schools in Fukushima Prefecture.
It will be based on the maximum allowed radiation per year for pupils,
at 20 milli-sievert/year.
20 milli-sievert/year was the maximum permissible dose to radiation workers averaged over five years, with a maximum of 50 mSv in any one year (see wiki), until Fukushima I Nuke Plant accident happened.
From Yomiuri Shinbun: (4/10/2011):
The Ministry of Education is about to set the standard for the amount of radiation when children engage in outdoor activities in kindergartens and schools, such as in the school yards.
In Fukushima Prefecture, relatively high levels of radiation and radioactive materials have been detected in some schools, and the Ministry has decided it is necessary to set the standard soon for deciding whether to allow outdoor activities such as physical education.
同省などによると、基準は、児童生徒の年間被曝(ひばく)許容量を２０ミリ・シーベルト（２万マイクロ・シーベルト）として、一般的な校庭の使用時間など を勘案して算定する方針。原子力安全委員会の助言を得た上で、大気中の線量基準などを同県に示す。基準を超えた場合、校庭を使用禁止にし、授業を屋内だけ に限るなどの措置をとる案も出ている。
According to the Ministry and other sources, the new radiation standard will take into account the typical amount of time when the schoolyards are in use, and will be based on the allowed radiation amount of 20 milli-sievert (20,000 micro-sievert) per year for children. After receiving the advice from the Nuclear Safety Commission, the Ministry will show [among others] the radiation standard for the air to the Fukushima Prefectural government. One idea is to prohibit the use of school yards and conduct classes only indoors when the limit is exceeded.