THE second semester of Israel's school year opened this week, but none of the country's 100,000 college students showed up for class. They are not truant; it is just that there is nobody to teach them, as a lecturers' strike enters its seventh week and threatens to torpedo the academic year. The professors and lecturers are demanding a 100 percent salary increase over the next three years. The Israeli Treasury, which bankrolls the universities, is offering only 30 percent over three years. With inflation running at about 10 percent a year, that leaves the teachers with nothing. Finance Minister Avraham Shohat, however, is reluctant to give in to the professors, fearing that a large pay raise would have provoke similar demands throughout the civil service, leaving his anti-inflation policy in tatters.