IsSorted

template IsSorted(n, b) {
  signal input in[n];
  signal output out;

  var acc = 0;
  for (var i = 1; i < n; i++) {
    var isLessThan = LessEqThan(b)([in[i-1], in[i]]);
    acc += isLessThan;
  }

  signal outs <== acc;
  var outsIsZero = IsZero()(outs);
  out <== 1 - outsIsZero;
}

If we need an array to be sorted, we could instead sort the array out-of-circuit and pass in the sorted array, finally asserting that it is sorted indeed. To do this, we can simply check that consecutive elements are ordered, that is $a_{i-1}\le a_i$ for all $1\le i<n$.

AssertSorted

template AssertSorted(n, b) {
  signal input in[n];

  // accumulator for in[i-1] < in[1] checks
  var acc = 0;
  for (var i = 1; i < n; i++) {
    var isLessThan = LessEqThan(b)([in[i-1], in[i]]);
    acc += isLessThan;
  }

  acc === n - 1;
}

If you would like to assert that the array is sorted instead of returning 0 or 1, you can simply check that acc === n-1 at the end. This is because we make $n-1$ comparisons and accumulate all of them within acc variable. If all passes check, that should sum up to $n-1$.