Gakuen Senki ~Kyoufu no Puppet Keikaku~

In late 23rd century was marked by a terrible disaster - the Third World War. Years have passed since the countries affected by it started slowly rebuilding. A league of nations was formed, consisting of "companions" from different countries. Suzuki, the representative of Japan, was also the founder of the most prestigious school of the new era - St. Innocence Girls High School. The graduates were invited to a grand opening held by a company that was successful in manufacturing giant robots. But terrorists of an unknown origin assaulted the place, killing most of the people. All the clues lead to St. Innocence school. Suzuki sends an agent (codename "Osamu") to infiltrate the school and find out what is happening.

[From Mobygames]