Is yellowish discharge considered impure in all schools of thought, and is it obligatory to perform istinja' (purification with water) after it, or is wiping with tissues sufficient?

Normal secretions are pure, but yellowish discharge (صفرة) is impure, and one must perform istinja' from it upon being certain of its emission. If you doubt whether it is yellowish discharge or normal wetness, the default is that istinja' is not obligatory. The opinion of the majority (jumhur) is more cautious regarding the obligation of istinja' for yellowish discharge.