Text Mining - Similarity among words to determine thresholds
Have you tried word2vec models ?. One of the main application is computing similar words, as they are very closer in the feature vector space. Thus apart from High_school and H_school, also secondary_school, public_school may also be obtained as closer words. The important thing is that a huge amount of sentences are needed for this unsupervised learning model.

Ref: gensim, word2vec in java