''I am sitting in the fifth class,'' wrote the 11-year-old Anne. ''We have no hour-classes we may do what we may prefer, of course we must get to a certain goal. Your mother will certainly know this system, it is called Montessori.'' She signed the letter Annelies Marie Frank. Margot, Anne's older sister, then 14, also described life at home and school and did refer to the threatening political situation, although without expressing alarm.

''We often listen to the radio as times are very exciting, having a frontier with Germany and being a small country we never feel safe,'' the handwritten letter in blue ink on lighter blue paper says.

The recipients were two sisters whose ages matched those of the Frank sisters. Betty Ann Wagner was 14, and her sister, Juanita, was 11. In a telephone interview yesterday, Betty Ann Wagner, who now lives in Burbank, Calif. - her sister, Juanita Wagner Hiltgen, lives in Redlands, Calif. - recalled the events that have heaped significance on an innocent exchange of correspondence.

The Wagners, whose home was on a farm in Danville, started the pen-pal mail at the suggestion of a teacher who had visited peacetime Europe in the summer of 1939 and collected the names of children her students might write to.

''We never received a reply from our second letters in answer to theirs, but we thought that had to do with the war and censorship,'' Miss Wagner said. ''After the war, in 1946, we wrote to the Amsterdam address that the letters had come from and we received a sad letter from their father, who told us how the family had died during the war.''