The basic difference in two sentences
KA121 is accreditation for schools that want to carry out recurring mobilities every year without needing to submit a separate project application. KA122 is a one-off project suitable for schools that are just starting with Erasmus+ or want to try a specific topic without a long-term commitment.
KA121 — accreditation for experienced schools
Schools with KA121 receive annual funding based on an approved European development plan. The advantage is predictability: you know in advance what resources you have to work with.
Important: KA121 is not suitable for schools carrying out fewer than 3 mobilities per year. In that case, KA122 is more efficient and administratively simpler.
KA122 — the ideal starting point
One-off KA122 projects typically run for 12 to 24 months. Many schools use this route to gain their first experience with European mobilities and build a network of partner institutions.