My situation: Two weeks ago I changed from fulltime employment to parttime (20 hours per week) to finish my bachelor's degree. I communicated this to my team leader and colleagues 4 months ago. My team leader agreed, and shortly afterwards the CEO and I signed the necessary agreements. It's an official state supported education plan that includes payment from the state to reduce financial impact, and it also states that I'll go back to fulltime 8 months from now. During this time I cannot be fired.
My company & team: We are about 20 people, divided into 4 departments. My direct team is pretty small and consists of 4 people including myself and the team leader. We have a loose hierarchy, and there are roughly summarized 2 different responsibility levels: Either you are responsible for a project, including requesting resources of all kind when needed, and reporting to the team leader in weekly meetings; or you work for other projects, basically being the human resource requested by a colleague. Roughly spoken, everybody in our team is a mixture of project leader and technician. Our team leader is pretty chaotic in terms of planning capacities, especially human resources. He often has no idea about the actual workload in our team. When a colleague of mine quit last summer, I tried to counter this with pro-actively showing a resource planing of my project hours for the remaining year. This plan showed capacity peaks of up to 160 % in almost every month, which of course is not doable, unless working 60 hours a week or more. My team leader responded with arguments like "The schedules will shift anyway, so don't worry", and from my perspective did not take my concerns serious.
... and finally my issue: I requested my part time 4 months ago, well knowing that resource-wise this would have a negative impact on my department and the workload of colleagues, especially since a colleague quit just half a year ago and the schedule delay that my team leader prognosed seems likely to hit in the next weeks to months. It feels a bit egoistic and I'm having a bad conciousness, but it's all about my future education and career, so I did it anyway. Nonetheless my team leader agreed, apparently without a proper adjustment of resource planing for our team, and without feedback of my colleagues (which would have possibly been a "No, we need him fulltime"). A senior colleague leads a project where I happen to be on the technician side for almost 2 years now, doing the productive work, and so far everything worked. He now expects me to keep the original schedule for the finalisation of this project (end of April), which would’ve been even tricky working fulltime, but at least possible. He gives me the feeling that now I'm responsible for reaching all milestones in time, which is not right from my point of view. He also knows that it's unrealistic, and when he asked me how I plan to finish this in time, I told him "I don't know" and "I expected our team leader to plan this properly", but we both know, that this won't happen. We didn’t discuss this since then, which is quite unpleasant for me. I played with open cards all along, and in my opinion I'm not responsible for projects where I'm just a "planned resource". All other projects I'm directly responsible for are in time and running.
[TL;DR] Question: Do you have any suggestions on handling this situation and stating the point to my colleague, that I don't feel responsible for meeting deadlines when I'm not responsible for the schedule and resource planing? In my opinion it's his task to adjust the project planing from the point on he knew that I would be available for just 20 hours a week (instead of 40). Am I even right, or should I reconsider my position?