Hundreds of teachers protesting the lowest pay in the nation began wildcat strikes that closed three Mississippi school systems in the first widespread teacher walkouts in state history. But plans by the Mississippi Assn. of Educators for a statewide strike by its 13,000 members were halted when the state obtained a court order blocking a walkout for 10 days. There are about 26,000 public school teachers in the state. They are demanding a $7,000 salary increase over the next two years to bring their pay up to the Southeastern average. Teacher pay in the state averages $15,971--lowest in the nation.