A little over 40 years ago, Roland Karlen bought his Upper West Side townhouse at 34 West 94th Street between Central Park West and Columbus Avenue for $25,000. Today, he’s watching his neighbors sell off their homes for around $6 million. Blame it on gentrification or a school’s eager board of directors, but Karlen’s block has become a hot ticket. His home is the last of nine on the block that hasn’t been sold off to Columbia Grammar and Preparatory School, a private institution founded about 250 years ago that has been looking to expand its holdings on West 94th Street. While Karlen and the school have an amicable relationship — the school headmaster said that it has no plan to muscle the retiree out — the school has reportedly offered top dollar for the properties on 94th Street. According to Dexter Guerrieri, an appraiser and head of Vandenberg Real Estate, the school has been offering above-value buyout rates for the homes.