Though we all instinctively avoid it, failure is inevitable in life. That's why learning resilience and how to incorporate feedback is a key part of any child's education. Truly failing is also instructive in the understanding of consequences: not doing homework (or forgetting it) should mean getting a poor grade on that assignment. But the feedback loop (and a student's learning process) may be short circuited by a parent bringing in the forgotten assignment, or a teacher extending a deadline.
Of course, following through on rules and deadlines can be difficult for a teacher who doesn't enjoy seeing students struggle. No teacher, really, wants to see students frustrated or thwarted by bad decisions. And parents too, can find it difficult not to intervene when their child is failing.